Don't know what your price range is, but here's a partial list of companies that make stereo preamps with HT passthroughs:
Adcom, Pass, McCormack, Sonic Frontiers, Rogue, Levinson/Proceed, VAC, AR, Conrad-Johnson, PSE, and probably a few others I've forgotten.
One thought about going with tubes in an HT system, even used as an HT passthrough you'll be burning the tubes. If you watch a lot of HT it might be a consideration as you may be replacing tubes more often than you might like.
Also, you don't necessarily need an HT passthrough -- any preamp will work, you just need to set a reference volume level on the stereo preamp and you're good to go. That would open up a world of choices. Best of luck.