It would be interesting to see what the results would be if you switched the sequence of cables around next time in another study of your perception of amp temperature and performance, ie, starting out next time (with a cold amp) with the "plain black cord" for the exact same duration of time you used the Pangea cable last time (as the first power cable used). Understand what I’m saying? If the same results take place, maybe it’s just the amp venting some of it’s heat to the heat sinks after being powered up for a certain total length of time (using a combination of both AC cables), no matter what AC power cables were used, and in what sequence.
I don’t know the entire design of the amp, so I don’t know if all of the operating current flows through the front panel ML-23.5 ON/OFF power switch, or if the front panel switch operates an internal relay, and all of the AC power/current then flows through relay contacts. From over the years arcing, some AC switches that pass high amounts of in-rush current can develop dirty contacts. It’s possible that maybe working the front panel switch a few times cleaned the contacts on the switch. These are just a few of my initial thoughts, but I do find your discovery interesting.
It would also be interesting to measure the resistance of each cable, with a test meter (measuring one leg/conductor at a time), to see if there’s any kind of difference between the two.