I personally think that the additional power in an 845 SET is not necessary. But, I guess that if someone wants to power some less-efficient multi-driver loudspeakers, it would be useful
What I would ask about this is, why would someone want to use a SET amp, and then drive inefficient multi-driver speakers with it? Perhaps I'm in a fog here, but I thought that the purpose of a SET amp was to provide coherence and purity of tone and harmonics. If you take the amp's output, and chop it up into bits and pieces in a phase-shifting passive crossover network which also causes other distortions, and eats up power and detail, use different drivers which inherently have different radiation source points, sonic characteristics, and dispersion patterns, and introduces odd-order harmonic distortion to a previously(primarily) even-order signal distortion profile, are we really getting what we wanted to get? I don't think so. But we will get what approximates a reasonably flat response test curve, as long as the output impedance is significantly enough below the speaker impedance curve to avoid reactive effects on the amp's response curve(not likely). What we really get with this kind of combination, is the relatively high even-order distortion profile of the amp, added to the even-order and also the easily discerned odd-order distortion profile of the multi-way speaker, making system distortion higher than necessary, and also in content(odd-order)which spreads out all over the spectrum, instead of primarily at the 2nd harmonic. In the case of lower efficiency, lower impedance(4 ohm) multi-driver speaker sytems, we can add the practically total loss of damping factor and cone control, and making the amp's freq response curve fluctuate with the speaker's reactive impedance curve, and add this to the inherently low internal damping ability of the less-efficient speakers, all causing bloat, flopping around cones, no tonal accuracy, and wild frequency response, added to all the other problems. Why someone would want to do this is beyond me, except maybe that they are after some deeper bass response at the significant expense of purity in the rest of the spectrum.
IMO, an efficient higher impedance single driver system is what is best with a SET amp. In that case a small 45, 2A3, or even 300B is fine.
If you want to say the 845 is better for deeper bass, fine, but I contend that it is not enough power for deep bass, and probably would require a powered subwoofer or powered woofer anyway for that purpose(another seam).
The issue is what is the goal of a SET system. Is it to simply provide a Class A amplification source, or is it to be something more than that? That's for the user to decide.
IMO, a SET amp can be much more than just a Class A amp. It can provide a distortion of predictable and less-invasive profile, that can be effectively utilized to work in a complementary fashion with certain loudspeakers, to create an overall system distortion profile of primarily easy-on-the-ear even-order harmonic distortion located in a fairly narrow range of the 2nd harmonic area, and nearly eliminating any of the problematic odd-order distortions which are so much more discernible to the ear that only extremely small amounts of it can be tolerated. Additionally, with the "certain"(single driver) speakers, it can deliver the sound from the amp without the inevitable multitude of problems associated with mainstream multi-driver speaker systems. Sure, single drivers have issues too, but delivering pure coherent high-detail sound with mostly even order distortion products at one area of the spectrum is not one of them.
My caution is thus, when you make a decision for a component, is it not a good idea to make the most of what it can do(as a whole), instead of trying to "shoe-horn" it into a more conventional role? I think so. The conventional wisdom of choosing a higher power amplifier, in order to make a wider selection of speakers possible to use(multi-driver), actually can lead you into a snafu which would not have been easily stepped-in with a lower power SET amplifier. The very fact that the low power SET amps practically forces you to use a high-efficiency single driver system(or horn) is actually a good thing, and not a limitation. The low power of the amp is actually almost "forcing" you to make the right decision. When higher power is available, then you can step right into the crap, unless you know what you're doing, and why you're doing it.
If you want to use an 845 SET, and you like it, that is fine, and there are some really good ones out there. However, there is alot more to a SET amp than how many watts it has, and whether it provides Class A amplification. With this type of amp, we can go beyond the mainstream, and get more than is typically available from more "normal" amplifier designs, if we study what they do and how they interact with various loudspeaker designs.
If you take a look on the web, at an article called "Why Single-ended amplifiers?"" by Audiopax's designer and engineer Eduardo de Lima(http://usuarios.uninet.com.br/~edelima/REASONS.htm), you can see his testing, graphs, and descriptions of the interactions that I mentioned above. He even shows how careful phase alignment of the speaker and amp output can work to reduce overall system distortion to below what any other type of package can produce, by using this type of combination.
I know this is a "radical" concept, but there can be more to things than easily meets the eye.
