McIntosh 402 amp channel stopped working

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Use your cell phone 3.5mm headphone output to the MC402 rca input, play some music to diagnose.

Knollbrent, I also have the 402 and use an Anthem D2v that I run fronts via ARC Ref5se thru unity gain. I see you tried running mono and both channels worked, but you don’t mention if you used Balanced or SE connection. Have you taken the cover off to inspect the connections and see if a wire has come loose on the inside of the inputs? Do you have a voltage/ohm meter to check continuity on the inputs external to internal? 
I have had a number of Mc amps and pre-amps and have never had issues with jacks but it a possibility. I think you problem is minor since you have confirmed both channels work in Mono. 
Call McIntosh tech support, I have always found them to be extremely helpand accomadating. I can’t speak for them but I have sent three items back to them, both out of warranty and was only charged for shipping. 2 came back in brand new packing. 

Good luck

Just got off the phone with McIntosh. Going to try a couple more things, if that doesn't work, I'll take it to a local Hi end shop

So I took the 402 into a local shop that is McIntosh tech certified and he fixed my amp and my Aloia CDP. Got the amp home and the left channel is still not working. Switched the IC's around going to the pre and discovered it was a faulty cable according to him. I through in another cable and it worked!

But, it's happening again. Same problem as before, left channel is not working. Could it have something to do with my preamp???

it could just be oxidized connections, use deoxit on all rca inputs/outputs and speaker outputs terminals. 

Have you tried switching L & R speaker cables on amp to see if problem switches channels?  Can you hook your CD player up directly to amp to eliminate a preamp issue?

Yes, just hooked up cd and using the 402amp, I ran everything thru my Anthem receiver and the two channels work fine. It's the preamp. C2200 is at least 15 years old and the tubes have never been changed. 

So now I have a second pair of IC's go bad within a week. It must be the preamp that's doing that?

Turned out 2 sets of IC's went bad within two weeks of each other. Same brand. They were sent to manufacturer and brought back up to speed. I LOVE Audience.