Here my experiences regarding SET amps over the 8 months or so and things I've learned. I have owned 3 845-based SETs including my current Consonance 845 monoblocks, which drive Hyperion speakers, and also have a Dared 2A3 amp driving Zu Druids.
I don't really understand some of the "big-tube" naysaying in this thread, as IMO the 845 produces some of the most pure, sweet mids of any triode. The 2A3 is nice, for sure, but I can tell you that my 845 system sounds better than the 2A3 (and it should, given other factors). The 845 IS a DHT so I don't see any real distinction here other than the power.
Regarding drivers for the 845: these Consonance amps do use 5687, and they sound VERY powerful on 90dB, SET-friendly speakers in a large room, with excellent dynamics. One might say the Chinese don't know what they're doing using the 5687 here, but deHavilland uses 6SN7s to drive their 845s which, I'm told, have even less voltage swing. This is probably more detail than you're interested in now (and this stuff is a bit over my head as well... I just know the amps sound very good, and big). It should be noted that Cary uses a 300B driver to push the tube into A2 for massive power; this is another thing altogether. And many claim that has a sonic penalty. I don't know.
The first time I heard a SET system was a revelation for me and the first SET amp I had in my home, even more so. So, I really want you to encourage you to try this, but those who are pointing out the issues with your speakers are probably correct. Now, I have driven DynAudio monitors less efficient than yours, and well, with a 22W triode push-pull amp, but most SETs are different due to the damping factor, which means high reactance with big impedance dips, as others have pointed out.
Regarding bass in general with SETs, I can tell you that it is EXCELLENT with the Hyperions with the 845s and excellent as well with the little 2A3 amp on the extremely efficient Druids. By this I mean that it is not underdamped, or wooly, to any extent for me; I find it entirely realistic and pleasant.
I happen to believe that SET magic comes largely from two areas: the lack of a phase-splitter (simple circuit another benefit) and the total lack of negative feedback. The best push-pull amps I've heard were also 0 negative feedback: AES Six Pacs. These amps also had rather high output impedance (poor damping factor) and yet also had great bass. Usually, a really small amount, 3 or 6 dB, of NF has little audible penalty. But NF is one of the biggest areas where what measures better sounds worse, nearly guaranteed!! NFB kills air and dimensionality and truncates decays prematurely. Once you hear 0 NFB amps, you finally realize you were missing some of the music.
Well, that's about it for now. As for 'advice' - It is fairly likely that you'll have to deal with some loose bass using SET on your current speakers, unless you high-pass them and get a sub. If you do so, I think even 8W would be sufficient power. However, the tube in that range - the 300B - I happen to think sounds inferior to the stuff both below and above it in the power range. At least a lot of the time.
