The experience that I can share—and that hopefully will be of some benefit to others—concerns just how sensitive a speaker should be in order to capture what @mglik calls “SET magic.” I repeated the same mistake multiple times (meaning with multiple speakers) over roughly 15 years—clinging to the belief that a speaker with a sensitivity rating in the low/mid-90s was plenty “SET friendly.” I didn’t realize how wrong—practically and conceptually—my belief was until I started experimenting in the high-90s. Then low-100s. And now approaching 110 with my current system (which is a multi-amped 5-way with single-ended amplifiers driving 4 of those channels).
Ultimately, the mistake I kept making was having no appreciation for what @atmasphere pointed out above: this amplifier topology shines when it’s not stressed, and the “20-25% of full power” figure sounds right. On the flip side, @mglik described some telltale sounds of an over-burdened SET amp: “dirty,” “colored,” “soft clipping.” I don’t think it has to be that way.
There’s also a somewhat hidden corollary, here, which is that I ended up multiplying my error by saddling a low power / high output impedance amplifier with a 12” or 15” woofer. The SET amplifiers with DHT output stages with which I’m familiar haven’t been powerful enough or “grippy” enough for this duty ... and that has led to anemic bass. Outside of some highly-specialized single-ended designs, the benefits of push-pull topology shouldn’t be overlooked, here.
One thing that might be beneficial is to get to know (but not necessarily acquire—restrain yourselves) some classic speakers from the era. (WE and Altec for starters.) These were, quite literally, “speakers designed to work well with SETs.” Open-minded listeners tend to come away from this kind of experience with a first-hand appreciation of the possibilities—and a very firm resolve to figure out how to capture some of “that sound” without inviting an albatross into the living room.
With that, I don’t mean to imply that there are no valid recommendations earlier in the thread. There may be—but I personally struggle to connect with this stuff absent any context. Listening space (a bedroom? a barn?) is super-relevant, here ... as is potential for a powered subwoofer, upstream component specs, listening habits and preferences, etc. For whatever reason, single-digit WPC and a mere handful or two of parts seem to place an increased premium on excellent overall system matching.