Using a good preamp with headphone amp worth it?

I have a small headphone setup for use when I can't use my main rig, but I'm finding that I'm using the headphone rig more and more. In this system I'm using a small passive preamp because I have more than one source and I also share it with a small self-powered sub/sat set. The preamp is fine, but it's not as good as going direct from source to headphone amp. The headphone amp is a good one -- the Grace 901 -- and my headphones are very good as well (Sennheiser HD600s and Audio technica W1000s). I also have a backup Stax set-up that I had packed up to sell, but put back just to be sure I want to do that. I run the headphone amp through the tape out on the passive. My passive has two pairs of outputs, but using the tape out avoids having two volume controls in the signal path.

Here's my question: In this case, would using a high-quality active (possibly tube) preamp be worth it? When using the tape out is the electronics of the preamp bypassed (especially with tubes)? In other words, is it not worth it to have a high-quality preamp if the source signal passes through the tape loop and into the headphone amp? If I were to use an output instead of the tape loop is having two volume controls in the signal path going to negate any added benefit of having a high-quality preamp (the Grace has a very high-quality volume control)?

The preamp currently serves primarily as a source selector, but it also serves as the volume control for the sub/sat system (which is of decent quality, but certainly not hi-fi). Someday, I may consider using a pair of small "bookshelf" speakers instead of the sub/sat (adding a power amp of course), but I think my wife will continue to prefer the diminutive apppearance of the sub/sat).

If you have any recomendations for preamps, here are my needs:
* at least 3 sources
* tape out
* preferably 2 outputs (to power amps)
* prefer diminutive appearance -- small and/or low height

Thanks.