Best temperature for optimum tube performance

It takes very little air flow to cool a chassis significantly. I've been doing it since the 70's. A couple of 12v 60mm PC cooler fans running on 3.5v are completely silent and cool the internal chassis from 50°C to 30°C on my tweeter PrimaLuna PL-5. Ditto a 100mm on midrange VTA M-125s.
PL-5: http://ielogical.com/assets/WinterBlues/PL5_Fans.jpg
M-125: http://ielogical.com/assets/M-125/BindingPost.jpg - note fan under tubes at front.

Thank you one and all for weighing in...
As an aside, I drilled a series of 1-1/4” holes in the upper back of the cabinet to provide convection venting....the two independent top drawers hide those holes.
I did install a small, quiet computer fan but it just couldn’t pull enough air to keep the gear from cooking.
(I was petrified to see millercarbon had replied.... I remain yet unskewered...love to witness the dust-ups he provokes.)
The temp gun is a great idea....

I use one of their component fans with front exhaust on my Parasound 5 channel SS amp. They have other fans for cabinets.

https://www.acinfinity.com/quiet-cabinet-fans/

Wow, those fan solutions are very inexpensive. Imagine if they were sold by Synergistic et al?

I can only add this... I have an Audio Research D160 tube amp from the early eighties, 1982 to be exact. It uses 6550 tubes biased to 65 mV, according to factory specs. Way to hot for any practical life. I have "detuned" them to 46-48 mV to calm things down while maintaining proper gain.  The amp came equipped with two fans to bathe the tubes in cool air.  And naturally, per the owner's manual, the amp takes a couple hours of use to achieve thermal equalibrium. Use this to your benefit. The detuning and the fans really help in this specific example.

Happy listening