This, from a guy who had the best sounding system I'd ever heard.

One thing I'm discovering about MY system is that "distortion at loud volumes" is a very EQ-dependent thing. Before I ran through my RTA passes, I'd EQ'd everything by ear, and the system was able to get quite loud without distortion, although I wasn't totally happy with the sound. Now, I can't get it as loud before getting distortion on certain material, but it sounds much better.

My next goal is to find exactly which frequencies are distorting and see if I can notch-correct those in the equalizer.

It's possible that on your system, you could do a similar thing. Find out what frequency is causing your loud-volume distortion, and notch it a bit with a very high Q. Remember that beta 11 has a bug with the Q where it only works on one channel if you adjust it in locked LR mode, so you have to adjust L/R separately if messing with the Q.