Most desirable vacuum tubes for SET amps

Interesting comment, maybe why so many like the sound of the Elrog 300B which feature the tungsten filament.

+1 @atmasphere Thanks for your insightful summary. It does seem that the lowly 1.5 watt 45 tube is becoming increasingly popular.

I do have a question for you, though. When you talk about the distortion that SETs have, you also state that if they are listened to below their distortion threshold (not exactly how you put it) they are very linear. Am I correct to understand from your comments that a SET listened to at very low volumes is not distorting? That would explain my experience, which I appreciate is limited... I listen typically in the near field well below 80dB. My speakers are 94dB efficient, so I *think* I'm typically using a fraction of a watt. I know that drum hits and crescendos can require the amp to produce instantaneous peaks far above that... But I don't seem to hear much or any distortion in my system. 

^^ At very low power levels the distortion of the amp is very low- hence that 'inner detail' you hear so much about. As the power increases distortion rises with it. If you are nearfield yous speakers may be efficient enough that you're not getting into trouble with distortion, but usually a 94dB speaker isn't enough.

Unless you're clipping the amp you won't hear outright distortion. At power levels less than clipping distortion imparts a tonality and at levels above about 20% of full power, a sense of 'dynamics' due to higher ordered harmonics present on transients.