I added a tubed preamp to my HT system for music and did just what Slartibartfast suggested. My Sony C333 SACD player is connected directly to the tubed preamp. The main L/R channels from the A/V processor, a B&K Components Reference 10, are also connected to the tubed preamp. You can also accomplish this goal by connecting the tape monitor out to the tubed preamp; my understanding is that the signal passes through the A/V preamp virtually untouched.
The Anthem Pre 1L and 2L are both tubed preamps with a processor loop - kinda the opposite of what you asked but they were built with your situation in mind.
The Anthem Pre 1L and 2L are both tubed preamps with a processor loop - kinda the opposite of what you asked but they were built with your situation in mind.