As Mr. Arnopol pointed out, heat is not the best thing for semiconductors. But there is more to it than that. Semiconductors die from crib death (I believe this was about 1/50,000 in the 80's or 90's, usually DOA or dies within normal factory burn-in), exceeding the thermal rating (not running at the high end of thermal ratings, but above), over-voltage (includes static discharge/ESD damage from poor handling, even before the part reaches the factory, often mistaken for a crib death, can make it past burn in, lasting, days, weeks or moths... the walking wounded, common with obsolete/out of production/surplus parts which are needed to keep old designs alive), or just old age (IIRC 20+ years typical, here is where keeping stuff cooler can help extend the age, but probably N/A to bassists... do you plan to keep your amp over 20 years?). <---My use of "(" ")" keeps this from being a run-on sentence. By the late 90's, I walked away from electronics, service and support, jumping out of the pot and into the fire, so I'm sure things have changed and my memory is fogged, but that should be the basic theory of it, and does not include such malpractice as counterfeit parts and user abuse (not beating the users, we all want to, but the user abusing the gear).
I believe the expectation that parts will run warmer or you will make the amp work harder at 2 ohms is false or at least a moot point for MI amps. For SS/class D, there are two types of amps I am aware of, those with a switch for 2 ohm operation, offering the same wattage at 2 ohm as 4 ohms (like GB IIRC or Mesa... nice as you don't need two 4 ohm cabs to get full power) and amps without a switch (requiring a 2 ohm load to get to full power, kind of nice for me, allowing me to detune performance for weaker cabs/lower volumes in my living room).
Amp with a 2 ohm switch: Moot. You have a D800. You have a pair of 4 ohm cabs, you play 1 cab and have around 800 WRMS, or you use both cabs, flip the switch and still have 800 WRMS available. Same temperature, 800 WRMS = 800 WRMS. Maybe you turn down and run cooler.
Amp without a 2 ohm switch: Maybe moot. You have a Carvin BX1600. You have a pair of 4 ohm cabs, you play 1 cab and have around 400 WRMS, or you use both cabs, and then have 800 WRMS available. Maybe you turn down and make it moot. Or maybe you just use both channels at 4 ohms and get the same power to both cabs at half the rated capacity making it moot.
Either way, if it's a good design, it won't matter.
Amen!
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