Can someone tell me what wrong

Could it be the speaker cable?

I think we can help you, by the process of elimination you describe, but your description of each change you make needs to be clearer and more specific.

For instance, what's the difference between swopping the monoblocks and hooking them up to opposing speakers?

In each instance we need to know what is hooked up where, and if there is any noise from either speaker.

For instance, when you hooked right mono to left speaker, did you also hook left mono to right speaker?  Was there noise is either channel?  Which speaker cable did you use?

If you reed the post he did swap monoblock

"swap the mono block - still the same noise"

Ok in order try these they cost nothing.. The way a mechanic would kinda do it..

1. Noisy valve, ay? You have a tube making noise? (Swap valves left to right for a check)

2. The mono blocks aren't on the same plug? Make sure they are plugged into the same rail on the meter. For now the same wall plug, or maintainer will work.

3. Make sure the IC (source to preamp AND preamp to amp) are tight, clean, and plugged all the way in. 

4. Get a cheater plug for the ground. Eliminate the ground with the cheater, ONE amp at a time. Repair the ground loop as needed. add a ground wire between the two amps, chassis to chassis.

You gave a good report, OP a lot better than most..

I'll give it a guess, a noisy valve, or a ground loop issue between the two monoblocks..

Happy hunting.

Regards