I have a Rivera Clubster 25 Doce 1x12 tube amp. Lately, when I'm playing the clean channel the volume will spontaneously cut by about 1/2. The dirty channel is not affected. If I switch the amp to Standby, the volume will very briefly increase just before it goes to standby. All tubes were just replaced and biased so I don't think there's an issue there. Could it be the FX loops jacks are dirty? I run effects in the FX loop. Could the Standby switch need cleaning or replacing? Ideas?