I realize I may be coming in late on the conversation...but I have 2 90XA's also, and I've had the sub-amp issue more than once. I was able to send one of my sub amps to Snell when they were still in operation. They sent me a "refurbished" sub-amp in return, that had been repaired. Now, I have an issue with the opposite sub-amp; it either hums very loudly, or it blows the fuse when it first fires up. Yes, there is a fuse; it pops out just below where the power cord plugs in the back of the speaker/amp.
I'll have to resort to finding a local electronics repair shop and hopefully find a schematic out there somewhere...