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

Call Upscale Audio.

Could be a bad tube. You might try switching the tube set from the right channel to the left channel and see if the problem follows the switch. BTW, did the protections circuit activate when you heard the pop?

BTW, I did not overlook your comments about switching 'some tubes' I was recommending you switch all tubes. If it were a tube, IMHO, it would probably be a small tube, unless you were tripping the bias circuit protection when you heard the pop. Another question, is the buzz superimposed over the (sound) signal or does the right channel just buzz? Anyway, switching the entire set of tubes should show if it is a defective tube or not. If not, pack it up and send it back to PrimaLuna for warranty work. FWIW, assuming you bought this new and have the packing I wouldn't worry much about shipping - the tripple boxing and Styrofoam lining are fairly bombproof. 

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.