As others have said, room correction software can help a lot. Personally I use Audyssey in a Denon receiver to do it, hence my somewhat odd soundcard choice. I do HDMI out to the receiver, it handles the room EQing and does an amazingly good job. ARC is Audyssey as a plugin. If you just need it on two channels and in a DAW, it is a cheap and easy way to get it. If you need surround, or want it on all sources, then you have to get a receiver with it and use HDMI. That is pricey, expect to spend $700 at least for one with MultEQ, and more like $2000 to get one with MultEQ XT32 like ARC has. Audyssey is worth it though, it is the first and only room correction I've heard that I really think does a good job. If you want to go the receiver route, look at Denon, they use Audyssey, have good hardware, and allow you to select a flat curve, as well as a re-EQ for cinema (some receivers only do a cinema curve).
Past that, acoustic treatment helps a lot depending on how much you are willing to do.
The Foam Factory is a good place for fairly economical wall stuff. Maybe not quite as good as Auralex, but good enough and pretty cheap. Only thing it doesn't handle is bass.
Bass is, of course, the really difficult thing to deal with. Multiple subwoofers help (not for volume but for uniformity) as does bass trapping. However, to work well, bass trapping needs to be huge.
GIK makes some of what you really want: Big traps you stick in the corners, floor to ceiling. That will do more to smooth out bass response than anything else. Pricey though, and of course they dominate the corners.
If you go nuts and bass trap all the corners, coat the walls with foam and diffusors, have a thick carpet or rubber (like you find in a gym) flooring, and then do some room EQ, you can make things pretty good (though not perfect). Of course doing all that will be a few grand at least. I'm in the process of redoing a room in to my computer/studio room and it won't be cheap.