PrimaLuna tube amp issue

Aloha,

I just received by mail a used PrimaLuna Dialogue Premium HO Integrated amp. I hooked it up and it sounded great but after about 45 minutes of listening, the right channel disappeared after a quiet pop.  I have swapped out speaker wires, cables...no effect. No bad tube lights come on. Left channel plays fine. Headphone only has left channel as well. It was running a little hot, but not excessively so. Any thoughts...

Bummer way to start with a new/used piece of equipment....sigh.

Thanks for your help in advance.

Michael