Yes, it is true that the tubes in a preamp will be “consumed” even when using HT bypass. Just keep in mind signal tubes in a tube preamp last longer than power tubes in a tube amp. 12AX7’s, for example, can last upwards of 10,000 hours. I’m about 75% 2-channel music, 25% multi-channel HT, so not a huge concern for me. If you’re the reverse, it might be more of a concern, but not nearly as much with a tube preamp as if you were looking at a tube amp with a bunch of KT88’s that you were going to be running 8 hours a day between music and HT. It was for that very reason that I went with a tube preamp and solid state power amp.
If the tube preamp has a true bypass, then it does not need to be powered on, and passes the HT signal through it.
As you already mentioned, 10000 hours is a quite while and should not pose a concern, even if the preamp doesn’t have a bypass… it is 1-10 cents/hour for a preamp’s tube cost.
