SInce the OP noted the RE xtx500.5 , i figured i'd write briefly.
It has now been discontinued, but can still be found at places for as low as 190 dollars or so.
I am very happy with it for a full-system amp. It isn't a powerhouse, so you should get _relatively_ efficient speakers- but i cannot complain about the sound or the size.
I mounted it to the underside of my passenger seat in my subaru forester.
My webpage has pictures of the guts of it compared to an old-school audio art 260.6xe, as well as pictures of the install, and other info about the amp:
www.c-j-l.net/Home/audio/subaru_stereo_installation_full]subaru_stereo_installation_full - Useful-Info
Enjoy!