Suggestions for a pre amp with HT passthrough?

Pass Labs XO.2 would be a good choice. No compromise 3-box design. Very quiet. Excellent pass-through with unity gain. I have owned tube equipment and got tired of fooling with tube issues (all of them have tube issues). Solid state is more dependable and good designs last forever.

I will agree that the Pass Labs X0.2 would be a good choice for SS. I will also agree that SS is more dependable and good designs last forever. However, after listening to tube preamps, I cannot return to SS.
So you have a good SS reco, my tube reco would be an Audio Research Reference 3 preamp. As quiet as any SS preamp I've heard, but sounds better playing music, IMHO.

Cheers,
John

Two of the more recognizable manufactures, Conrad Johnson and Audio Research both offer HT pass through on their tube preamps from their entry model up to the more costly ones. I’ve tried the AR SP16 and currently own the CJ CT6. Both due a nice job (I prefer the CJ sound to AR) and I never had tube issues with either one.

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.