Is it the amp or the wiring?

I'd be grateful for thoughts on a strange coincidence.

I had been struggling with my beloved Luxman L-450, which was refurbished and recapped by Jude Hercules in Brooklyn. It sounded great, but every once in a while the left channel would crap out. If I turned the power off and then back on again, sometime it would come back to life. Also, if I turned the selector to an unused input, then turned the volume way up, that would also sometimes bring the channel back to life.

I took the Luxman back to Jude, but try as he might he could not reproduce the problem.  So I put the Luxman aside and subbed in my Rega Brio.

For a while, the Rega worked great, no problems. And then . . . the left channel started crapping out! Even stranger, I could get it going again by turning the amp off and then back on again. I had been using the Brio for years in a different setup without this problem.

So two amps in the same setup, both having the same problem fixable by the same solution.

That suggest that the problem is not the amps but the setup. So here is the setup: The source is an Auralic Altair and the speakers are Heresy IIIs, all bought new. Also, the speaker wiring is in-wall wiring. It was done by a competent electrician, but I am growing suspicious. Could that be the problem? What are some other areas to troubleshoot?

Have you ruled out an issue with your left speaker connection?  Literally the terminals and wire at the speaker?
Oh yeah. Very solid connection.
I'd also lean towards some intermittent or unreliable wiring issue going to the left speaker, or in the left speaker. That would be the first thing I'd try to diagnose anyway.
Just bypass the internal speaker wiring and see what or what doesn't happen. Then you'll know if it's the cause. Could be some critter chewed on the wiring in your wall.

All the best,
Thanks all. I'll try different wiring. But the thing where turning it on and off again fixes it temporarily suggests that it is not a loose wire but something weirder, no?
Yes, and, no. By swapping out the integrateds, you've eliminated that variable. Trying different speaker cables will determine whether your next step would be the speakers. Something could have slipped past QC at the factory, only to assert itself in your system.

Step by step is the only way to go.

All the best,
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Ah, clever and quick check. I will swap the cables connecting the amp to to the in-wall wiring left for right and see what happens! More when the results are in  . . .
Wiring could have a very small short that only shows itself when there is enough current-potential to over come the resistance in the short, that could show up as an intermittent problem. If your wiring is in your wall then only way to check that is a megger. Most electricians will have one in their multi-meters.  get another set of wires and try that, don't have to be fancy any lamp cord will work for fault finding purposes. switch speakers too to eliminate them as a culprit.