My ValveKing has tube covers (preamp tubes only) like the ones on this head:

Are these good or bad for the tubes? I would assume that they trap heat and would therefore reduce the lifespan of the tubes.
Epiphone G-400
GFS Crunchy PAF - Bridge

DigiTech Bad Monkey
iSP Decimator

New Amp Fund: Depleted
Those covers let in less noise. Some tubes are microphonic and will caputure som of the sound surrounding the tubes. I have one on my VM.

I dunno if all the tubes in that head are microphonic though. Some tubes are and some aren't. We'll.... they are all, but more so than others.
Epiphone Elitist Les Paul Custom WCR and Dimarzio equipped
Marshall 2266 Vintage Modern
Marshall 1960TV
Dunlop Crybaby From Hell