Acoustics is a pretty neat science. Just like no two cymbals will sound identical, even ones that were made on the same line, no two rooms will give you identical sound.
Everything from the dimensions, the location of the kit, the floor, what's on the other side, even the paint will have a certain impact on what frequencies and harmonics get squashed. Speaking of harmonics, this is what you are noticing an substantial decrease in as your sound is now a lot clearer in the room.
Drywall will reflect a majority of frequencies, especially highs. you could pin up a quilt over where your kit is and probably notice a substantial difference with just that. Or, put a few tiles of acoustic foam. You can also hang them from the ceiling like a sign.
I'm more in favor of changing a couple things in the room rather than totally retune your kit. Especially if you use it for live performances and such.
You can create more or less bass in a room by working the corners. Absorptive material in the corner (wedges) will lower the overall bass in the room, if you want more bass, you could try facing your kit in a different angle. I guarantee you will hear changes.
I'm not an expert on this by any means, more of a practician.
Give it a shot and let us know what you find.
