10 gauge power cord. Too much power for tube amp?

Sounds like some kind of in rush current problem. I don't know anything about your amp but I'd guess it's something in the rectification (wrong tube type?) or voltage regulators. 

You ever measure what's coming out of your wall socket just for curiosity sake? Mine gets as high as 125v certain times of the day..although that still shouldn't be an issue if everything is working correctly. I've seen people post numbers as high as 128v in older homes, which can eventually wreck havoc on older tube gear. Get you one of those Amprx Brown Box or their Linestage models from a place you can return it if it's no help. Works wonders on older tube gear and it seems like it cleans the power signal as well if compared to bypass mode. Cheaper options do exist.  Although I think your issue is a problem in the circuitry if not the Rectifier. I'm no repairman though. Definitely not a power cord issue. 

Generally speaking tube amps like 14 awg or 12’s.

The McIntosh model you're using is a monoblock.  A 300 watt tube monoblock.  So if you're turning two of these on at the same time and they're connected to the same 15 amp outlet that might push things a bit, especially if that outlet and its associated breaker isn't dedicated solely to those amps.  The manual says they draw 5.5 amps/120v, 6.6 amps/100v.  

Tubes seem very sensitive to ac quality and what you need is good power cable and a good power conditioner. 
 

I got weary of the day to day variable sound from tubed equipment and gave up. 
 

With all solid state now I have given up a bit of warmth and pleasant ambiance but have gained more detail and day to day consistent sound quality. 

@cundare2 The usual suspects (Fluke, etc.) make very expensive power quality analyzers. There are some EMI/RFI to audio toys made by a couple of people just for the audiophile crowd.

In the middle of the actually useful pack is a digital oscilloscope with the right probes. I found the frequency measurement features in mine to have been very difficult to use.

Lastly, you can build something that will let you input the AC to your audio jacks via a step down and isolation transformer/instrumentation amplifier. This is the cheapest solution that will let you use off the shelf PC scope software but requires the most amount of care in building to ensure you don’t electrocute yourself or your PC.

https://amzn.to/3FSuA98