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.
10 gauge power cord. Too much power for tube amp?
Tube amplifiers tend to be sensitive on incoming voltages. Is there a chance a bigger gauge power cord like 10 gauge may not be a good thing?
My amplifier tends to shut down occasionally upon start up. maybe moving to a higher gauge might be better. Does it matter?