Please explain preamp theater pass thru

In wanting to upgrade my system as a move toward more music and less HT, I have recently purchased Vandersteen 3a Signatures. I currently have a Classe SSP25 surround processor. I would like to add a tubed preamp for music.

After researching on the gon, it appears that a nice preamp with a processor loop, or theater pass thru as I've also heard it called, would benefit me in this upgrade. My understanding is that in bypass mode, the preamp becomes no more than a wire allowing the pre/pro to send its digital signal out without the preamp coming into play. Unfortunately, I have become confused on this issue as several people are telling me that any preamp will suffice by just connecting to any source mode of the preamp but having to make some adjustments to the gain since the pre/pro cannot control the volume gain in this configuration. Just doesn't sound right to me. Again, my understanding is that you want the preamp out of the signal path so that the pre/pro can do its thing. Occurs to me without a pass thru, the 2 channel preamp would send out its own signal to the mains even in HT mode instead of allowing the pre/pro signal to send its info to the speakers.

Ouch, my head hurts. Can someone (who really knows) please clarify this for me.

Mucha Gracias to anyone that can help...

P.S. I have another thread under the amps/preamps discussion section asking about recommendations on a good preamp for my system. Any suggestions on a good preamp would also be appreciated.