In most cases, you are going to need a lot more than something that will just "fit" in there because you need at least 2" above most receivers for proper ventilation. So unfortunately, unless you can cut a large hole a few holes in the back of the armoire, this is not going to be a good place to put a receiver. For somewhat smaller receivers that don't generate as much heat, you might want to look into something digital like the Panasonic XR series of receivers.
THX doesn't need to factor into the equation, but you do need something to do the pre/pro functions (decoding, volume control, source switching, etc...). Basically all receviers will do this for you.