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

Sorry to hear. Especially since Prima Luna is on my very short list of amps if I were to replace or upgrade mine. You've made all the right moves to narrow it down to something internal, like caps or transformers or wiring. The 20 minutes makes me think thermal. But if you turn it off and right back on again (no cool-down) and it still goes for 20 minutes wow, you got a mystery there. 

I've shipped enough of this stuff to no longer worry about the component. What I worry about now is the box! Cardboard gets beat up. 

Get yourself the Costco rolls of packing tape. Fold cardboard into angles and tape them to reinforce all the edges and corners. Then cut another box to fit snug and tape the you know what out of that. So a box to protect the box.

Print FRAGILE GLASS and laminate that to every side. Then call around, or ask your shipper, to stick shock sensors (liquid filled tube breaks if hit too hard) on it. 

Thanks everyone for your tips. It's not the tubes.  The amp plays with no issues for 33-34 minutes , then a pop then buzzing in the right channel only.  I bought it preowned .    Absolutely blown away by the way it pushes my Focal Kanta No 2's.  It worked great for about a week. Upscale Audio tech guy is going to call me back. I just wanted to thank all of you , I don't know how to respond to individual posts. 

Craig 

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.