DONT listen to Kuda or Cuda or what ever the hell it was. Your right, subs work off the air inside the box. Shallow mount subs could help... depends on how much volume of air will be left over. I'd try to get a ported box, if the one you have isn't already ported. The ported box will allow more air, obviously, into your box and possibly get your subs bass back. As for the amp, only get one if yours isn't matched up with your subs correctly.
Just in case you dont know how to match them up heres a quick lesson: Take the TOTAL RMS power of all your subs and purchase an amp that has the same RMS. And make sure to set your gains correctly. I've also heard somewhere that if you put layers of cotton in the bottom of your box it will "trick" your subs into thinking theres more air space and creating a deeper bass.....but I've never tried it so I'm not sure if its even true. But good luck!