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Yeah, they probably use it as part of a power supply for something relatively minor, entrusting sound quality (where it matters) only to solid state chips and the like. I don't think tubes would be my first choice in a (electrically) noisey car environment, but hey..

It really is part of the audio path. Alledgely you can tell when it's on because everything sounds "warmer". There is an AOpen motherboard with a tube doing similar things as well.

There is an option to bypass the tube. I guess you'll need that for when the tube has killed itself because it's been bounced around too much...