I own three (yes, three) Rock Crushers. I have found them to be killer on my less gainy amps like the 100W 12000 series clone and the non-KS 100W Marsha. They are pretty transparent (to me) and can actually figure into the "final sound" ie, how much power section distortion, how compressed, etc, and um, how LOUD the amp is when the RC is inline and attenuating. It has settings (warm, edge) that change the tone curve as well, and they are useful, really adding to the tone controls on the amp. Your settings on the amp may change a bit with the RC engaged, but there is no loss of feel and responsiveness IMHO.
However, on my "new" KS 100W '79 JMP (THANKS J. B.) the RC works best at -3 or -8 db. After that it appears to quash the throaty tone produced by the amp, particularly through the HBE input. It's odd but the RC seems to "genericize" the JMP by subtly altering the tone curve and responsiveness of the amp to your playing. And that's not the best result, given the character of the KS sound. That said, it is still a critical component to me because the master volume goes from off to ON very quickly when using the HBE input and the SAT option. I mean the thing gets loud in a hurry. So with the RC at -3 or -8 I can play to my heart's content without worrying about other family members not hearing the TV or keeping my neighbors awake. And the special, special tone of the HBE/SAT/C45 Friedman JMP is preserved, at least to my ears. This amp sounds like the Olympian Gods roaring in unison, and produces a tonality I have only heard on other KS mods and Marshas. It's too beautiful for words - you must hear one in the flesh to fully understand the sonic nature of these amps.
Honestly, I would have gone with the Faustine but the $1K price tag and the 1 year wait killed that option for me. I see it this way: I saved $1.5K by going with an almost as good alternative, which gives me funds for another head. Which of course means I will have to buy a fourth Rock Crusher.
Matt