strange issue with 3rd channel of Dual Rectifier

Just picked up a multi-watt Dual Rectifier, and everything works great except for channel 3. At low volumes it functions normally, but when the volume is turned up it has this hard limiter thing going on. The first split second of the note is normal volume, but then instantly drops to whisper quiet. I've tried various combinations of modes, wattage, and rectifier, and it doesn't matter how that is set. If the volume goes over a certain threshold (not all that loud actually), I get that limiting effect.

Never had a problem like this with an amp, so I'm kind of at a loss as to where to start trouble shooting. Anybody else ever been faced with this issue? Any other info I can provide to help figure this out?

Thanks for any help!

Have you tried this, from the FAQ section:

Q: My amp gets quiet and then loud (rolling volume). What could be the cause?

A: Check to see if your Cathode Follower preamp positions (typically V3 & V4 on most Rectifiers) have Russian tubes. If they do, try installing some Chinese preamp tubes. 90% of the time, this will solve your volume roll issue.