Well, I played with this some more last night. Sadly, I was not able to reproduce the problem I have described where I get no change when trying to trim the sub value via remote. Indeed, I measured pink noise output from 0 to +15 dB with the volume control vs 0 to +15 dB with the sub trim control (all other speakers OFF). There was no difference with my digital RS SPL meter.

So it looks like I made a mistake. The sub trim is working in my system on 2-channel stereo modes. The issue of the all sub attenuations combining to be greater than -15 dB still exists, but the trim seems to be working.

My thanks and apologies to the Outlaws!

-- Otto