So I had Nik include a passive serial effects loop in my 2204. It works great with some pedals, and I get a lot of 60 Hz hum with other pedals. The pedals it hums with are not known to be noisy pedals (Deluxe Memory Man, Redwitch Titan Delay, even the Klein-ulator!). Pedals that do not hum (or very slight hum) are my MXR Carbon Copy and Fulltone Mini Deja Vibe. When I unplug from the FX loop (or jumper with a cable from Send to Return) it is dead quiet. I've tried so many things I don't think I could list them all, but here is a summary of what I've discovered:
1. It's not my cables
2. It's not the relative position of my cables/pedals to my amp
3. It's not a ground loop nor does it seem to be a problem with the signal level. I purchased an ebTech Line Leveler to knock down the Line Level Output (+4dBu) to instrument level (-10 dBV) and cancel ground loop hum. In fact, this device hummed without any effects plugged into it - just the LL into my FX loop hummed.
4. I purchased the Klein-ulator FX loop buffer from Nik. It hums when in my FX loop with no pedals plugged into the Klein. This is exactly the problem the Klein is supposed to cure.
5. It's not my power supply for the FXs or the Klein. I even tried using different wall outlets for the amp and FXs with no change. I've tried my isolated outputs Pedal Power 2 Plus and also my transformer-less Power All with no change. Still it hums.
6. It is not volume-related. I turn up the amp and the hum is still present at the same level.
7. It's not my pre-amp, or PI or power tubes. I've changed them all with no change in hum.
8. It does all the above without anything plugged into the amp's input.

So I'm fairly frustrated at this point. I'm fairly certain it has to do with an impedance mismatch between my amp's Send/Return and what I'm plugging it into. Again, this is the purpose of the Klein and it doesn't seem to be working with the 2204. It may have been designed for the OTS but it should have a very similar impedance requirements.

Any ideas/suggestions/comments are much appreciated!
Thanks!

Yes, I did try using different outlets/mains for the amp and FX power supply. Still got the hum. I can understand having a noisy pedal once in a while but when the FX Loop Buffer (Klein) gave me 60 Hz hum ... I'm not sure where to turn. Thanks for the reply.