For what it's worth....and opinions on this vary.... I find that the Archon (and many other amps with a half-power setting) actually sound better at 100W with the master turned back than they do at 50W with the master turned up to provide the same output volume. You might try experimenting to see which you like best.
Also....and again opinions vary.... I find that high gain amps like the Archon get more of their character from the preamp, and the tradeoff between a lower master volume and no attenuator versus a higher master volume with an attenuator favors the "no-attenuator" case when comparing at the same overall speaker output level. This isn't true for some other amps of course. But the Archon, set at 100W, is one of the best "bedroom" level amps I have, with no attenuator and through a 2x12.
You might have different preferences though, so by all means, experiment with it. The rockcrusher is a fine attenuator.