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.