If you prefer a one liner suggestions, just feel free to skip the rest after the following sentence:
Assuming your B&W diamond N805 D4 signature speakers are the 805 diamond D4, I would say replace the NAD AVR and then see what happens.
If you don't mind more details:
I have no experience with any of the exact devices (but somewhat familiar with the brands and related products) you listed), so I would offer the following bullet points based on my understanding of the specifications and available bench measurements only:
- Speakers in-room FR are most important, the B&W N805 D4 is probably fine, assuming it is the 805 D4 that has the diamond tweeter, I am confused with the "N" because the older B&W non diamond models used that letter a lot, ie. N for Nautilus but I am not aware they use it for the diamond series.
- The PS Audio amp is likely not popular here or on ASR, due to their not too good measurements relative to many other amps at the price point or even at much lower price point, but your M700 does have more than enough voltage and current for the 805D4 so unless you can return them or trade them in for something that measure better, I would say just keep them and upgrade the AVR/AVP first and see what happens.
- At -20 or even -15, if you can hear distortions, then I am sure you are right, it won't be due to the power amp or the 805D4, though it might be possible if your heard it from the R200C and/or the R8s.
- NAD T758V3 - That's 100% the weak link, based on published specs and the ASR bench measurements. It is probably had the worse measurements on multiple fronts relative to all other AVRs/AVCs/AVPs measure on ASR.
So, it would seem if you want to upgrade, you should target the NAD AVR first. The T758V3's poor measurements do not mean they will sound bad, they can, and should, still sound very good especially if you run Dirac Live properly, but it does depend more on you listening habit and speakers. In your case, since you are only talking about volume at -20,
I don't think it is the issue, but it is possible that you are very sensitive to low level distortions such that even at 0.5% THD+N, it may bother you.
If that's the case, any midrange D+M AVRs, or Yamaha's A6A/A8A should solve the problem, or the NAD T778 if you want Dirac Live included (for D+M you have to buy the DL license separately).
If your dealer pushes Anthem, then I would try to get him to give you a good trade-in value for the NAD. The AVM90 does seem like an overkill. From specs and measurements, I would say the AVM70 should be as good, unless you need 4 independent subouts. Owners of the AVM90 will tell you the significant better "sound quality" you will get from the 90, vs the 70, but I would bet 10:1 that in a blind listening test, none of them will be able to tell a difference between their 90, and the 70, or any of Anthem AVRs, yes, even the lowly MRX540, but that's a different topic and the debate on blind tests vs sighted tests will never settle on forums. I have listened to the 90 in the dealers demo room for no less than 30 minutes, to me there was nothing remarkable about it's sonic performance.
