If you really believe its a heat issue go out and buy an indoor/outdoor
thermometer with the little wire outdoor probe. Rest that probe on the ventilation holes with the amps at idle for 24 hours. Take reading.
Put on some music that you are sure you can hear the difference after
the amps have warmed up, when they become "right" take a reading.
After your listening session let the amps cool back to idle, take reading.
Now block the ventilation holes with a suitable small throw pillow or
whatever. Monitor the heat build up and see if it approaches the "right"
temperature. Note time to proper temperature. Remove pillow. Listen again and see if that cured the warm up issue or the warm up time is less.
As Hunter Thompson said "I don't recommend blocking vent holes, but
its always worked for me." Use caution.
I did this years ago with a Hafler amp, used a pizza box cut and taped together to form a cover for the whole amp. YMMV.
BTW, this is a fun and cheap experiment, way cheaper than switching a bunch of components.
Let me know how it works out.