Can temperature fluctuations affect audio gear?

HiFi.
Big is correct. Devices are apparently designed with a max junction temp at a given air temp. Lower air temp is gravey.

Read at least the written description of the math. It'll make sense. The heat-engine model doesn't apply where greater differences in temp produce greater results. transistor temps are sort of self-limiting and there are bunches of design criteria.

That being said, cooler is still better.

Sitting by a window with outdoor temp of 50F,little transfer
Sitting By a window with outdoor temp of 0 F,great transfer,
and it takes place both ways.At 0 F high rate of heat out,at same time,high rate of cold transferring in at 0 F.The more extreme, the faster the transfer rate.

As near as I can figure, that's the whole point. You'll have a zero difference when you fire it up with everything at room temp....BUT as soon as the device warms, it'll start moving heat away at such a rate as to maintain the calculated (or nearly) temp diff above ambient.
If you stick your room temp amp into the freezer, same deal.....just that now your working against the freezers ability to pull heat out (pump it) 'uphill' into the room....creating the cold box. Amp doesn't know this and will still end up warm VS the internal cold box temp.....and the same # of degrees.
If you put a power transistor in an insulated space and power it up,
you'll have a meltdown / failure in no time. No place for the heat to go. somewhere above maybe 150 to 200c, it'd just cease to function. These limits are all straight physics and chemistry.


But, based on the math the device will shed heat at a certain rate. Once it's been on and is stable, it'll run pretty much a certain amount above ambient for a long time. Even in the freezer......

This is pretty clear from the link I posted.
Also, find a datasheet. Somewhere is the 'derate' for power devices...and maybe others. The derate deals with power and temp. And how much less the device will take and at what rate, as it warms.

The OP? Long gone, but still and all, I'd not run my gear in an 85f space. Any weak link will be ruthlessly exposed. Any dry heatsink compound which isn't doing its job....a nut/screw securing a power transistor has come loose. A dust bunny clogging heatsink fins or some venting,.... All can hurt your stuff.

http://www.ee.latrobe.edu.au/internal/workshop/store/pdf/MJ2955.pdf

First datasheet I came across.
Look at figure #1 for temp derate and the bottom of page #1, just above the fig for the heat/ thermal #s.

That strikes me as a good datasheet to use for purposes of focusing the discussion, Magfan.

It's interesting to note, extrapolating from the data that is provided, that its maximum rated power dissipation of 115 watts at a case temperature of 25 degC/77 degF (commonly referred to as "room ambient") does not drop off by a factor of 2 (to 57.5 watts) until the case temperature has risen to 112.5 degC/234.5 degF! And it can go considerably higher than that, as well, if the power that it is called upon to dissipate is reduced such that the junction temperature is kept below the rated maximum of 200 degC/392 degF!

Some related things that should be kept in mind, though:

1)The numbers provided are "maximum" ratings, commonly referred to in other datasheets as "absolute maximum" ratings. Those are the ratings which if exceeded stand a good chance of causing immediate failure. A good design will provide a very large margin between those ratings and the actual operating conditions. As noted in the reference you provided earlier, a rough rule of thumb is that each 10 degC reduction in junction temperature doubles mtbf.

2)"Derating" can refer to two different things. It is used in the datasheet to refer to the falloff in MAXIMUM power handling capability that occurs as case temperature increases. "Derating" is also used to refer to the amount of margin that the design provides between the rated maximums and the actual operating conditions.

3)The amounts by which both case temperature and junction temperature rise above ambient temperature will depend on the adequacy of the heat sinking that is provided, and on how much power the circuit application requires the device to dissipate.

Best regards,
-- Al