With more testing of the amp and some other testing of various outlets on my house I am pretty sure that the problem is a ground fault in the mains of the house itself. By cranking my old practice up high I found that the same basic noise was present in it as well, and I carried it around the house and plugged it into every outlet, including the outside outlets. I think it must be a ground fault near the electricity meter somewhere as it affects everything.
The good news is that it seems that the amp is fine. I think I am hearing more noises in it because playing it in a house I cannot crank it up to even jam room volumes, and haven't had it up past 2, and I don't even know if I've had it quite that high until tonight. Tonight I went to a friend's band practice session, and he A/B'ed my 6534+ against his old 5150 II, through his old 5150 cabinet (both ten years old or more). We set the EQs on exactly the same settings. We did not try out all the channels because I forgot to take my footswitch and his is very temperamental and wouldn't work with my amp (his has been repaired several times). Anyway his settings are very scooped - low and high dimed and mid on 1 - with the gain, presence, and resonance cranked on the lead channel. The other guitarist played through his Krankenstein head and Krank Rev 1 cabinet. Anyway this is what we observed:
The 6534+ has a much more defined mid-range than the 5150 II, even with the huge mid scoop. It also has a lot more cut than the 5150 II vis-a-vis the Krank set-up in a band context and the high end was clearer. This might be due to the newness of my amp relative to his, but he has recently serviced the tubes in his amp so that might not be such a factor.
We didn't use any pedals - not even my ISP Decimator - and it all sounded fine in a band context.
Anyway I am pleased that the amp is sounding good when cranked up to louder volumes. I really think that the issue is that it has been stuck on the lower volumes up until now. I have a jam session with friends on Friday this week but I'm playing with someone with a 20W Laney Lionheart and it is in a house with minimal soundproofing so I won't be able to go past 2 on the post gain but it should still sound better than what I have been getting so far at home.
I also learned that the Peavey 5150/6065 cabinet sounds good with the 6534+ - I can definitely recommend it for death metal and thrash stuff. I left my Orange PPC412 at home, as I will this Friday (then I will have to borrow my friend's 4x10 bass cab - that should be interesting!). It is less middy and directional than the PPC412. Depending on your application I think it can be a very good cab.
Also I let my friend play through my Explorer for a few songs after tuning it to D standard (the tuning for his band). He was very impressed with the Warpigs and plans to buy some BKPs for a new project. It did indeed sound massive through his amp. He didn't try playing my guitar through my amp.