PRIMA LUNA DIALOGUE PREMIUM - 20 minutes and then loud pop and buzzing in 1 channel

BTW the buzzing can be drowned out by greater volume. 

I own the PL Dialogue Premium Integrated amp and experienced something similar but not the same.  It DID turn out to be a bad 12AU7. PL suggests a specific process in my manual.  When I first switched a few tubes it wasn’t in the logical process PL laid out. 

My problem was at startup, just before the protection circuit clicked in.  Low hum rising in volume until there was a pop. No damage to the speaker.  After that, no problems listening. 

Following the PL process, the problem switched to the right channel, I marked the tube bad and ordered another.   All is good now.  

Just sharing an experience. I hope your problem is as simple.  
Primaluna is an outstanding amp.   BTW, I’m running Spendor A6, not super efficient, but PL drives them just fine.  

If the problem stayed on a channel after you switched tubes and/or cables then it might be a blown cap.

I have a PrimaLuna Dialogue Premium HP that drives a pair of Magico A3s cleanly and with authority. I love this integrated amp. No issues at all. I would definitely, as suggested above, get Upscale Audio involved. Good luck and please report back as to what the resolution to the issue was. Thanks. 

could be tubes, could be caps or anything. I heard a popping noise like that on my guitar amp it was a preamp 12au7. On my hifi was an old telefunken. I have to recap the whole amp. My lesson is I gotta have a backup amp in a situation like this so I bought a used solid state. Its not the same but at least it got me by and got my amp done the right way. Now it sounds like thunder!