I would add the Tube Research Labs Samson monoblocks to the list you are considering. These amps are not as small as the Nuforce, Claytons, or Channel Islands amps, but they are smaller and lighter than many of the Class A behemoths such as Plinius and Pass Labs. I have an ordinary rack, and they fit comfortably in it, albeit not side by side. They run absolutely cool to the touch, and the sound is utterly astonishing--to say they have "serious control of the bottom end while being musically correct" is an understatement.
Grannyring wrote an outstanding review on these amps that I recommend and agree with. Needless to say, I'm deliriously happy with mine.
Good luck in your search!