When I was setting up my home theater I tried multiple HT receivers and all were unsatisfying, though I wasn't very high in the quality chain admittedly. I ended up going the passthru route as Macdadtexas suggests. It's by far the most convenient and most economical, and also gives you the best rig for HT. Also, you needn't run out and find a preamp that has passthru. You can use any available line-level input on your existing 2 channel preamp (assuming you have one available). Connect the front L/R preamp outputs from your HT receiver to this preamp input, then calibrate your your HT output levels for a pre-defined preamp volume level. For example, set your 2 channel preamp at 12:00 and calibrate your HT. Each time you go into HT mode, simply select the input for HT and set your preamp at 12:00. Your receiver front L/R amps go unused.
This setup makes no compromises for stereo sound quality, and in your case will likely benefit your HT sound by virtue of the tube amp on the front.
The only flaw I see in this for you is you may not want to burn up tubes while in HT mode (my rig is in 2 channel mode most of the time).
One thing to be careful of if you go with a speaker switch or swap cables like Blindjim recommends - don't power up your tube amp without speakers connected unless your amp has a protective circuit against that.