Four problems.....the xover, the bass, the waveguide/driver and the frame (I guess that is everything...hehe):

The crossover ....the Behringer unit stock is not very transparent and has tons of stuff in the signal path.....it really needs to be seriously modded to really hear this speaker.  Please see our Behringer mod page for more info.  Modding the xover makes every single thing better....bass, midrange, treble, imaging, palpability, purity, dynamics....everything.

The Bass :  Doing these 5 things below results in WAY better bass, dynamics, speed and lower midrange/midrange purity....remember that the woofers need at least 200 hours of LOUD playing to finally loosen up.

The woofers really need to be seriously damped.  The frames ring like crazy.  I got Clayton to do some damping but it is not enough.  You need to use a constrained layer damping material like EAR SD-40AL.  Cover all the struts on the back of the bass drivers. 

The woofers should have a little felt on the inside of the frames installed to stop some of the higher frequencies hitting the frames and bouncing back into the cone.  You can use simple black cheap felt and glue it on.  Too much and it might Sound too dull (according to one listener).

There should be no grill clothes on the front of the woofers.  The material on the stock Emerald Physics speakers is very dense and veils the sound......remove it!

There should be no grill cloth material on the back of the drivers.  This way you can hardwire the woofers directly from your amp, bypassing the crappy binding posts and ordinary Kimber hook up wire used.

You must run a brace up the back of the speaker and mount that brace to the panel and both of the magnets on the Alpha woofers.  This makes a huge improvement to the whole sound. 

The waveguide/compression driver :  The stock waveguide and compression drivers are,  well.....bad!!! ....however, the waveguide can be saved!. 

The waveguide rings like crazy......you need to damp every inch of the back of it with constrained layer damping......ie.  EAR SD-40AL or Dynamat Extreme......way, way better sound. 

The stock waveguide has defraction problems.  You need to get some thin felt and cut it in a strip about one half inch wide and long enough to go all the way around the outside of the waveguide.  It needs to just overhang the "edge" where it transitions to go flat to mount to the baffle.  You can use double stick scotch tape all around the waveguide and just fold the felt around and stick it on (picture coming).  This makes the imaging mucho better and makes it purer sounding.

The waveguide has a threaded portion on it that screws up the sound.  Whatever driver you use should be mounted so the waveguide throat is transitioning purely to the driver with no long tube for the sound to go down.  You can cut off the brass threaded part with a hacksaw, file and sand it flat and the glue the driver on with Amazing Goop and once the glue has set use some SD40AL strips between the face of the tweeter and the damped waveguide (picture coming)......much more open, fast and pure sound this way.  These waveguides are available at Parts Express for $12 each so if you cut one too short, you can get another.  Buy a few before trying this mod.

The stock compression driver is bright and hard sounding.  You can substitute the B&C DE250 driver (used by Earl Geddes in his speakers), the Beyma 385 or other good driver (you do not want a driver with threads on it....you want a driver that is flat that normally mounts to a horn with bolts).  It will sound way better.  Also you can use a Vifa D27TG-35 dome tweeter (pictured below) that costs $30 and cut off the very back (only) of its damping chamber (you must leave the felt in the back of the tweeter!!!) and the sound is quite amazing.  This modded tweeter sounds as pure as a $300 Esotar tweeter (I A/Bed them)!  The Vifa dome tweeter will lower the sensitivity to around 93 so a more powerful amp is needed if you play more than 100 db (a compression driver will give around 100 db allowing the use of even 2 watt amps).

Please hardwire the tweeter directly to the amp......use no connectors anywhere....way more pure than the binding posts and stock wiring.

Try some felt or foam inside the waveguide to kill the sound bouncing around inside the waveguide.  You can felt the whole inside of the waveguide or buy some 30PPi foam and glue it in.....please read this link         for more info on the foam.     OK....link does not work completely......so when on Audiocircle go to the open baffle speaker forum and look for the thread "DIY Emerald Physics clones" and open page three and you will see some info by Brassear about the foam.......more to come.

You can also buy the DDS 90 waveguide ($250 the pair....used in the Emerald Physics CS-1) or the Geddes Waveguides.....but these are more expensive options that will give you small improvements.....in fact, the smaller DDS waveguide is probably worse (cause it is smaller) than a properly modded Parts Express 12 inch waveguide ($24.00 for a pair)......Geddes waveguide is probably a bit better but costs a bunch ($400 each, yikes!!!!) and is no longer available.

The frame/baffle :  Needs to be strong and non-resonant at least 2.25 inches total thickness.  Constrained layer damping works great here.  MDF is not very good.  Use highly refined MDF (Medite 3D and Rangerboard are two brands).  3 pieces of 3/4 inch thick ultra refined MDF with Green glue in between each layer is very dead! Or you could use one piece of highly refined MDF in the middle and a super plywood on the front and back.  Use Green Glue (a damping glue) and a few screws to hold them all together.  Other combos using hardwood, aluminum, etc. can be used.  You must use a brace and if you are really serious you will want a vertical solid one inch thick marine plywood brace that runs at right angles to the front panel and screw this brace to the front baffle between the woofs and waveguide and to the base (see very crude drawing at bottom of page...will make a better one soon) and epoxy  this brace to the back of the woofer magnets so the back of the woofs is held super ridged. This makes a HUGE Difference. 

Build your own .  You can buy the stock woofers...the 4 Eminence Alpha 15As, a few 12 inch Dayton Waveguides, the Behringer xover and the Vifa tweeters from Parts Express.  You can buy all four woofers, a few waveguides, 2 Vifa tweeters and the xover for around $700 plus shipping.  Add some EAR damping material, Green Glue (constrained layer damping glue), felt/foam, Amazing Goop, hardware: including some good spiked feet, brass driver mounting screws and a couple of sheets of three quarter inch thick highly refined particle board and the cost is around $1000 or less.  The sound, if done as above, will be light years ahead of a stock CS-2 and give you goosebumps and chills.....yup....it is pretty outrageous how good you can get this thing.....of course, you will want the xover modded.....he he......I have no doubt that this thing done to the max will be sonically superior to the latest EP speakers, as well.  Better sound as your unit would be hardwired (no binding posts and no extra wire), have no veiling grillclothes, have better damped woofers, etc. plus you can make them look anyway you like.  What is so nice that it pretty easy to build open baffle speakers....way easier than boxes....and way better sound!

If you really want to shake the house then buy 8 woofers (four per channel) and use two on the front of each baffle and 2 more on a rear baffle with the rear woofers facing rear and wire them out of phase to the front ones (since they are firing rear they will actually be in phase).....series parallel will take you back to 8 ohms.  Use braces between the baffles to make the whole speaker rigid.  The rear baffles need only be as high as the two woofs.

All out three way version:  Use 4 woofs on their own separate baffles connected together as above, and on another baffle use a couple of 6 or 8 inch drivers from 100 hz to 800 hz and then use a waveguide above that.  One of the orginal prototypes of the CS-1 was done this way but Clayton decided that biamping on a smaller scale would sell better.....and have better WAF.

Below are some pics of Kevin's speakers using the modded Vifa tweet.  These first pics do not have the felted waveguide....shown in last pic.  He said the felting of the inside of the waveguide gave a nice improvement especially at higher listening levels.  Notice all the constrained layer damping on the back of the waveguide and woofs....very well done.  I will try to get him to take some closeups of the felting and where the tweeter mounts to the waveguide to give you a more clear idea.  Nice looking speakers.....even his wife likes them......that cake looks yummy!  He is using brass screws too.  You could black them before mounting so you would not see them so much.... however, some like the higher tech look the screws give. He has less than $1000 in them including buying a saw and router in the flea market. By far, the best speaker he has ever had and he has not properly eqed them, time aligned braced them nor had the xover modded....He is going to go crazy when these are done!!!!! The xover is 1000 hz at 48db per octave.  Bel-Canto Ref 1000s on bottom and soon to be Pass F5 clone on top.  You could probably go as low as 800 cycles with these tweets......tested very briefly that way.