I am going to add my $0.02 here for you, since I believe it to be solicited :)
I do not have experience with this amp, but I have been working with similar symptoms with my reference 2-channel system that includes a fully tubed input/output pre-amp (all 6SN7 tubes), and hybrid tubed mono-block power amps (6922s on the input stage only).
Here was my struggle:
When I first built this system, all I could hear was pops, hiss, and what seemed like the radio coming through the speakers. IT DROVE ME CRAZY after investing so much money into a HiFi system.... First I purchased a pretty expensive power-line conditioner to eliminate the EMF and RFI noise that existed; I hope it would be the silver bullet, but sadly, it was not. Next, I went out and bought a variety of pretty fancy tubes for the pre-amp. This again helped to a variety of degrees based on the tubes being rolled, with new-stock cryo-treated Electro-harmonix being the quietest I have found. Next, I realized there was noise being caused by mechanical resonances in the amps themselves, and therefore the tubes. So, I added sorbothane feet to the bottom of the pre-amp, and a medium weighted rock to the top, close to the tubes. This paid serious dividends in reducing the noise floor, but some noise remained...
Where is that noise coming from? The tubes! What have I realized? No matter what you do, there will always be tube noise! Why? Because the quality of my amplifiers won't hide anything, and tubes will always have noise, or they wouldn't be adding that "warmth" everyone is after.
Moral of this story is that no tube amp will be perfectly quiet, but if you are at 25%-50% gain, and hear a bunch of pops and crackles, you likely have crappy tubes, crappy power, or a mechanical isolation issue. A good quiet amp will make these three things even more evident than one filled with counter-distortion effects.
Sorry for the rant, but hopefully this helps you avoid having to do all the guessing and testing I went through to come to this realization.