Wow, nice preamp Rlf! I am sure that is a glorious combination!
I have spoken to a couple, perhaps a few that are using the Supratek preamps with the H2o's and are very happy.
I owned the First Sound Presence Deluxe Mk II prior to driving to Henry's and auditioning the H2o's with the Adcom GFP-750, Henry's "Fire" preamp, and my preamp I brought in tow. I personally felt my preamp easily "bested" the other two. As well it should with the price point differences taken into consideration as well. In all fairness, Henry was still tweaking his preamp, and I "hogged" the sweet spot when my preamp was in the system. I still wish I would have let Henry hear it from the sweet spot. Off axis listening is good for some areas and not for others.
I can attest the First Sound Presence Deluxe Mk II is a phenominal combination with these amps, however they are not all the inexpensive with a retail of $4200 and improve with a better after market power cord. They are also rare to find on the used market.
I would look for a used Supratek or First Sound here on audiogon if you aren't in any hurry.
LoL - disregaurd all of that, I just reread your post. You need HT pass thru and balanced. Scratch the First Sound as they are all single ended. I too think the Suprateks are limited to perhaps one or two balanced options.
In your quest I would have the Aesthetix at the top of my list. I am not sure how the Synergy IIi would do, an excellent preamp, but I think it may be on the cool side, and the H2o is very neutral - liquid, but not warm. Perhaps BAT preamps would have HT and balanced as an option as well.
Good luck and keep us posted.