Wow, great responses, here's what I am thinking.
Different speakers - I have considered this as a next step, and was thinking about Zu, Devore 3XLs, and Joseph Audio RM-25si, all of which I think would get my amp closer to the loafing so enjoyed by wolf_garcia's Carys. Any others I should think about? Maybe start another thread...
atmasphere, thanks for the math/design explanation, conceptually I knew this but the details are VERY illuminating.
erik_squires and testpilot, I would love to end up with combo of tube pre and ss or d power, but that's too far of a $$$ leap right now. I have also considered a Schitt Freya through a Krell 305il, an integrated that I don't use as I very much prefer the BAT with the Thiels. Low cost, low risk, plus I could finally try a BAT/Schitt system.
I am very much intrigued by the ideas of building a kit amp, but imagine I would need to tune up (learn?) some soldering skills and have some time set aside to do this, but if I did, I imagine I would go down the 300B route, as I was inspired by Herb's recent review of the EleKit, which in turn gets me back to high efficiency speakers. Eventually I do want to hear what the fuss is about with 300Bs. But I will look into the VTA. Bonus here is that if I built an amp from a kit, I'd have the confidence to rebuild the xovers on the Thiels!
And fyi, Underwood's recent offering of a Music Reference RM-200 mk2 (and Fremer's review, and my hunch that the current capabilities of the VK-55 are what makes it sound good with the Thiel's to begin with) is what got this ball rolling.
Thanks all - cheers and enjoy the music!