Which Speakers?

The Thiels seem like a natural considering your already using them for surrounds.

Agree with Unsound. Theil will integrate better then another brand with what you have.

IMO, you have already picked 4 very good brands. I would suggest you continue with Thiel just for continuity and whatever synergy it may bring. What type of amplifier do you have?

I've been swapping out a variety of speakers for the L/R in my HT - Reimer Teton GS (giants) w/no center, Paradigm Ref Studio 100's with Studio CC, Odyssey Nightingales, and finally my Merlin TSM-MX's w/o a center channel. Contrary to what I would have thought, the little Merlins used as "large" with "phantom" center are by far the most coherent of all of them, especially for dialog. I presume that's because they are essentially point sources in that mode. I wouldn't try them with a cheapo receiver, but in my Arcam system they work really well. And of course, for music, they are terrific. I have 2 ACI Titan II subs as well, so the foundation is no problem. If you go with any of those brands I'd suggest staying with a "small" 2-way like I did if it works out to integrate the sub for music. My family members actually demanded I put the TSM's back in the HT after swiping them for use in the separate 2 channel system.

Where you sit, where the speakers are, and what acoustic treatments you use are all going to have a _huge_ effect on what speaker works best for you.

Much more than the room size.

I have a Sherbourne Amp