ARC repair needed

Sounds like a thermal issue on one side which is invoking a protection circuit. Can you swap the speaker leads to see if the fault moves to the other side? If it does, it's a speaker or cabling issue. 

I tried swapping both input leads and outputs to speakers.  I tried reverse on the preamp and inverting in the amp.  It’s always the same output.  I suspect it’s either a solder or a capacitor.  

You can use freezer spray to check for cold joints.

High End Audio Repair in Phoenix is great.  James is the owner.

shipping power amps always an issue, for risk of damage as well as cost

george meyer av in downtown la is your best bet