So a rep from Rogue who I’ve been messaging back and forth with is also stating that he thinks this is "tube rush" and that while some amount is to be expected with tube gear, the amount I recorded likely indicates that the center 12ux7 preamp tube is wearing out.
I used a phone app db meter to get a baseline measurement for the noise I’m hearing. I set the amp volume knob at 12 o’clock and took the measurement from as close to the speaker as I could without actually touching it [~50 db].
I got hold of a new set of just the same tubes that the amp ships with because I wanted to keep any new variables out of the mix for now, and I went through, first swapping in just a new center tube [~31 db], then checking noise again, then swapped the two other 12ux7 tubes [~41 db, higher?], then swapped in two new 12ax7 tubes and checked again [~40 db].
So new preamp tube had a measurable positive effect, then new tubes in the other positions had a lesser but measurable negative effect. Unclear on why it went up but happy to just leave those other new tubes out of the system for now.
Then I needed to add some source components back to confirm that the the amp was the sole cause of the issue, which I tested the following day. Plugged a Bluesound Node into aux1, and measured again, [~45 db], so close-ish, if slightly higher than nothing plugged in the previous day. Then switched to the amp’s internal phono, which doesn’t have a source plugged into it, but is set to MC/100ohm [~50 db], then tried my Parasound phono amp plugged into aux2, also set to MC/100ohm [~55 db]. The Rouge rep seemed to expect this as the result of the phono amp’s higher gain and that seems reasonable.
So now, I feel like I need to put the bad tube back in the center and get measurements with both sources, but the improvement, while audible, is not as much as I had hoped for, and I guess I’m wondering what an acceptable amount of "tube rush" actually is in an amp at around this level. I’m unclear I guess on whether I’ve addressed the issue as much as I can, and the rest is just the cost of doing business with tubes, or if I should press the Rogue rep to help me further investigate?
Additionally, if it is actually a factor of the tubes themselves moreso than the amp (not their age, but specifically the make/model etc.) and this could be better addressed by using different tubes than the stock ones, I’d love anyone’s personal recommendations (within reason, I can’t switch to ultra-rare $200-a-piece tubes just to shave off a few more db of noise).
[There was also a fun moment where I thought I had somehow made the sound much worse, but it turned out that when I was putting everything back after cleaning, I had tried a different layout where the turntable was between the phono and amp, but this put the cartridge too close to the amp I guess.
It caused a very noticeable hum that went away when I moved the turntable back over to the left side and the phono in the middle.
I’m also going to try to move the phono amp inside the console on an internal shelf since it doesn’t get hot at all, and this would leave much more breathing room between the turntable and amp, but I have to cut a hole in the back to run the cords through, so that’s a job for another day.]