Emily- I'm not sure of the rationale behind the question. Assuming that the tube pre-amp provides you with what you perceive is better sound, why not leave the tubes in the circuit? Set the volume control to some easily repeatable position (straight up?) and adjust the surround levels accordingly. I'm assuming that you will use your primary audio amp for the front speakers in the HT mode. As for modifying a tube pre-amp, almost anything is possible if you have the know-how and the time. However, it would require adding a switch or relays in the output path of the pre-amp which could have a negative effect on the overall sound, combined with possible chassis mods to accommodate the switch shaft/knob or toggle switch(s) depending on implimentation. (I'd consider a double pole relay which, when the pre-amp was powered off, would allow pass-through of a particular input. I'd resist using an A/C relay and try to find an existing DC power source that could supply the necessary current needs of the relay(s)). In general, I wouldn't recommend it. Just a thought. Good luck.
