845 tubes for Bel Canto SETi40?

My Bel Canto SETi40 cuts out after about 10-20 minutes of playing 845 B (Valve Art) and 845 M (TNT) as well as a few other Chinese tubes. The sound just goes off and it can be reset by hitting the Mute button twice. But with regular Valve Art 845s, it does not do this and plays perfectly. Any users of this amp tried any of the above tubes or NOS tubes? Bel Canto have tested the amp thoroughly and tell me it is perfect and meets all specs. It worked in the factory for a few weeks with their stock Chinese tubes without a hiccup.
Any information or insights would be appreciated.
Let me try again - has anybody tried an 845B or 845M in a Bel Canto SETi40 or SET40 amp?
Thanks for your input.
No comment on your Bel Canto situation because I have Audio Note. Sorry state of affairs. Let us know how things turn out...good luck.

Can you share your thoughts on 845 tubes sound. I'm ready to buy some and I've been searching the web for the last two days. I was considering those 845M and the KR 845, but still have no idea how they will sound.
Check out the tubes asylum at Audio Asylum
Search Ed Sawyer and 845. He has some good reviews on the Super TNT 845's.