FWIW:
Bass depth and weight are experienced as very similar between the 2.7 and 3.7. Though there are tracks where you can definitively hear the 3.7 go a bit lower.
The main difference is the sense of scale and linearity. The 3.7 just creates larger sonic images and a bigger soundstage. What exactly to attribute that to is a bit of a puzzle to me as they share the same mid/tweeter and only differ slightly in woofer size. Is it JUST that extra 2 inches in woofer size? Or is it something about the bigger cabinet of the 3.7 as well? I don’t know exactly what causes this difference. Confusing the issue a bit further: with the brief tests I've done integrating a sub with the 2.7s, the soundstage does seem to grow somewhat, but I don't think the image sizes expand in to 3.7 territory - and yet with a subwoofer that's adding another 10" driver and producing more bass than the 3.7!
I’ve also wondered why the 3.7 sounds a bit more linear all the way to the bottom of it’s range. It could be a difference in voicing to some degree (maybe the 2.7 was voiced with a tiny bulge to compensate for it’s slightly smaller size - though I don’t think that was normally the Thiel way. Thiel was never shy about keeping linearity even if it meant their smaller speakers sounding a bit base-shy compared to some other brands).
I’ve wondered if the added control/linearity has anything to do with the different woofer constructions. The 3.7 has that sort of dimpled woofer and passive radiator and it’s general shape was claimed by Jim Thiel to get rid of some common reflections around woofers. So I wonder if that contributes.
