Just saw this - Maxmad is correct, all four smaller tubes you are asking about are the line stage, and the four larger ones are the power tubes. My understanding is that when you use the HT bypass, it bypasses the linestage, as the HT receiver is functioning as a preamp, so the smaller tubes would not be in use in that situation at all, but this could be incorrect. I have not yet used mine as part of a home theater system and don't plan to for some time yet, I have only used it for two-channel listening. You should contact Kevin Deal, I think his name is, at Upscale Audio, who is the US distributor of PrimaLuna. Please let me know if what I said here is incorrect!
