Is this a common problem..? I'm still building up but a rack heating up is quite a scary thought..
-- SeveredSerpents
The problem is mainly due to the power supplies. These are usually the primary heat source inside cabs; a few 4 hp supplies get hot enough that you'll see a warning on them about high temperatures. Most of the time, this isn't a concern...but if the heat builds up enough, it can throw modules that need precise CVs (VCOs, for example) for tuning reasons into fits. This has been a constant battle, actually. One of ARP's early adverts touts the stability of their VCOs as opposed to Moog's. Some of this was due to better design in the exponential voltage converters...but the original Moog 901s were notorious for janky tuning issues due to heat.
The other big problem is that excessive heat isn't good for electronic components. Over time, this can manifest as various instabilities in modules. The power supplies themselves are pretty susceptible to this issue, actually, plus they have to deal with their own current inrush issues on power up which, if components have been damaged enough, could cause you to need a new power supply.
What I would suggest, if you're worried about this, is to snake a wire into the cab for a temperature probe. Sounds super-esoteric, but it's not. https://www.amazon.com/Calibrated-Electronic-Thermometer-Waterproof-B60900-2700/dp/B00VA3I77Y/ref=sr_1_12?crid=EB1IQCGK27EC&keywords=electronic%2Bthermometer&qid=1639692887&sprefix=electronic%2Bther%2Caps%2C188&sr=8-12&th=1 goes for $26 and change. Once the cab is filled, tape the sensor inside the case so that it's held in place by tape across the wire (DON'T cover the probe itself!) while making sure that the probe isn't near any circuitry, fire it up, and see where the temperature winds up once things stabilized. If things rise to over 100F/40C, then you might need to ventilate the cab with some vent blanks or, to avoid losing panel space, follow mrsupersubsonic's rear case venting method above.