Best temperature for optimum tube performance

The simple answer is no. 
Always provide gear adequate ventilation.   Especially tube gear and power amps.  They generate the most heat. 

For not lowering my standards enough, I do apologize. Will try to do better, by which I mean worse, going forward.  

Not possible

The filament of the tube is intimately associated with the cathode of the tube which it heats. Room temperature isn’t going to affect them much at all so I wouldn’t worry about that. But a fan is a good idea because heat does wear tubes out, and it also shortens the life of electrolytic capacitors (which are used in the power supply).

Now there is an optimal filament voltage- because that is what controls the cathode temperature. But the lifespan of the tube should be considered also. If the filament voltage is slightly lower you’ll get longer life, just like a light bulb. But if its too low, you can damage the cathode. There are a lot of variables here and this is really the sort of thing that is up to the designer. But as far as a fan goes, its a good practice regardless of tube or solid state.

One of my better purchases for my system is this exhaust fan that’s built to be placed on top of audio components and is extremely quiet.. It has 3 fans that suck the hot air up and blow it out either the front or back depending on the unit. It has an LED temperature gauge to measure the internal temp of the component and you set it like a thermostat to the temp you want and it then cycles on/off to maintain that temperature.  I use two of them on placed top of my tube amps.  And did I say it’s quiet?  It’s brilliant. 
AC Infinity AIRCOM T10, Quiet Cooling Blower Fan System 17" Front-Exhaust, for Receivers, Amps, DVR, AV Cabinet Components https://www.amazon.com/dp/B078PQJSRY/ref=cm_sw_r_cp_api_glt_fabc_A0Q6TTEXXKMTGSYFS93T?_encoding=UTF8...