Well, I just posted to your other thread about kick drum phase problems. Here's a hint that I don't think people really do very often but makes a lot of sense to me and you can definitely hear the difference. If your overheads are positioned on the beater side of the kick drum, you will be picking up kick drums sounds that are the inverse (polarity) of what the kick drum mic is picking up. Same with any kick drum leaking into the snare mic. So, you might be getting some phase cancelling when you mix in the overheads with the kick drum mic that make the kick sound like crap. Try inverting the polarity of the kick drum with respect to the other 3 mics and see what happens.
Another thing that I bet is hampering your sound is that carpet padding on the walls. All that is doing is killing all of that nice sparkly high-end. Generally, a dead room is not just a room with no high-end, it has broad band absorption/diffusion. With the carpet padding on the walls (or carpet on the floor for that matter) you will likely have all the same problems with room modes, etc. you had before, but now you have no high end.