Sometimes it takes using a different browser for success; however, always a good idea to try LOADing the saved config.dat file immediately after the SAVE as the file may be corrupted and require being saved again.So after playing around with an (apparently slightly defective) x8500h for a little while, I'm back to the x7200aw. I'm pretty sure at this point the 16 years old subwoofer itself is the culprit (either a bad switch or some component having aged itself out of spec), it sporadically did work on the 8500 but it was just as likely to not work the next time around, kind of like with the 7200 before that. My testing of the subwoofer outputs on each receiver (with a small subwoofer) seems to confirm that the outputs are still functional.
While playing around, haven't yet been able to get my Mac to upload to the 7200 a saved config.dat file I created prior to re-initializing its firmware; will try a (also pretty old) Windows 7 PC in the next few days. At this point it's running with a configuration I did manually prior to trying out the 8500. One of the benefits perhaps of doing a full re-init is the Denon AVR Remote app sprung back to life again on my iPad (I noticed last night), yay.
My new sub should be arriving within the next week, so at that point I will do a from scratch Audyssey Pro calibration and see what happens. Selected a SVS SB16-Ultra to replace the old B&W 15 incher (ASW850). I'll be getting a workout next week that's for sure, as they are both quite heavy.