Hmm well those were the biggest things I could think of that would normally have a significant impact on sound quality, and it seems like you have most of that stuff sorted out pretty well.....
-8 MV is fairly loud but if your room is small, I would think the AVR
should have enough power especially in 2 channel.
In case you haven't seen it, here are some measurements on your speakers:
Monitor Audio Gold 100
Looks like something a little odd going on around crossover?
Maybe it wouldn't hurt to try a good 2/3 channel external amp? With the right choice, you could gain more power and a higher SINAD to see if that makes a difference? I'm saying this because it seems like you've covered all the other bases.
The challenge I see here is the X8500 is -103 dB from pre-outs so many amps are not this good. Also, the X8500 does have decent power so we need an amp with -103 dB or better and A LOT of power to be in improvement. I'm not sure what the best choice would be here.....NC500 has the power but is only -95 dB and I'd hate to not match or exceed the Denon's signal. NC400 would match up but doesn't have much more power if any.
Maybe Purifi or Benchmark?