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

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

 As others have mentioned, it is not the power cable. Also tubes are surprisingly robust and will tolerate more abuse than any MOSFET.

You could use a PC that's has a greater cross-section than 10AWG without any problems, in fact could reasonably expect a slight improvement, the amp will only use the power that is being demanded but there could be a capacitor in the power supply that is going/gone faulty and pulling more current (amps) than the protection circuitry allows. I have experienced a similar problem with the PSU bleed resistor going low ohms.

Perhaps do a visual inspection of the power caps, they will be positioned close to the power transformer, and look for any dicscolouration, swelling or residual smell.

You have already received probably the best advice:  Contacting McIntosh would be the sensible thing to do.