A persistent hummmm...

@OP. That kind of hum problem can be very difficult to diagnose. As you have found out by using two different valve amplifiers, tube amps are inherently more susceptible to low frequency hum - often at the frequency of the mains - 60Hz in the US. Tubes aside, I have known situations where certain amplifiers hum in certain installations and do not do it anywhere else, with no apparent reason. Though it would be a bit of a pain, you could move the amp and speakers to another room to check if there anything in the listening room that is inducing the hum. But before that, I would disconnect all electronic appliances in the house and see if the hum disappears. Then you can reconnect the one by one to see if you can identify a cause. It's a long shot because the PS Audio regenerator should deal with that, but in the circumstances, every option has to be explored.

Has the amp ever been quiet or did it have this hum from the first time you used it?

I’ve had hum from tube amps associated with noisy tubes. Atmasphere already confirmed you swapped the input tubes whch are more likely to be the problem. I’ve had full boxes of tubes that were noisy so it is possible the replacement tubes are just as noisy.

Have you used any connection improvement products? These are dangerous as they can create a path to ground.

Jerry

@dtorc 

I had a similar experience with a horn/tube combo. Do you have any dimmer switches nearby that can transmit RF? Also, a dedicated home run to the panel is recommended. Since you live in the “Boston” area, do you have an old house? Some electrical panel updates can cause ground loop issues with old BX wiring. 

I had that issue years ago. I started unplugging everything in the house until I found the culprit. A coffee machine. 

I can sympathize. A hum can get into your head until the whole musical experience is ruined. I too have a hot tube amp and my speakers boast of a 104 db efficiency rating. (Asking for trouble but I owned speakers before I discovered the amp) No matter what (and I mean, no matter what) the hum, though minimized, could still be heard from my chair between songs and even during songs if they ever took a breath. This forced me to stay on a certain type of music that maintained a clatter while I was trying to forget about the hum. Nearly ruined the whole experience forever UNTIL....I rearranged my room to be able to move 4 feet further away from my speakers. (Kilpsch Lascala II) Problem solved!! I can lean forward and put my head back in the "hum zone" but back 3-4 feet nearly zero hum. Why is this? I have many theories but no real 'scientific' answer. AND of course, very few can rearrange their listening room so as to gain both width and depth. I removed 10 bookshelves around and behind my listening areas to gain that extra space. This comes after a million other attempts. Yes. I had an electrician run two dedicated lines directly from outside power box to my listening area, Changed in and out tubes incessantly. Sent back to manufacturer to install grounding harness and new improved everything. And while everything helped a little, moving back has made it so weak I finally don't even think about it any more. A side benefit to all this isolation etc is that the quality of reproduction has also improved,