Which high power SET

Pani, don't agree with you. I have a VAC 300 watt push pull mono blocks and my Absolare 845 mono blocks are better in every way. Less mechanical, more open, and much more natural. The VAC is great don't get me wrong, but sounds a little veiled in comparison.

It hasn't been established that the Harbeth is incompatible, just speculation. Edoit is doing fine with his present 30 watt amplifier so a similar power SET is at the very least worth an audition. Either he'll like it or he won't. Listening is the only way to determine this. It could disappoint him or reveal a honesty and realism he's not heard before. Let the ears decide.
Charles,

Pani,
I appreciate your comments but I must agree with Jwm (Jeff). He and I have been good friends for many years and have heard numerous amplifiers in our respective systems the past 22 years. VTL, ARC, CJ, VAC and on and on. His 845 PSET is simply superior sounding and far more natural than all the previous amps mentioned in his home. The VAC IMO was the best of the push pull amplifiers he had. Even the VAC PHI 300.1 mono blocks were out done by his sublime performing Absolare Passion amplifier. Jeff like many others was skeptical about SET capabilities, not any more. Pani your opinion is just as valid as ours, the point is that we all hear as distinct individuals.
Charles,

Many have expressed that harbeths are incompatible with SET.
Is this a problem of topology (SET vs push-pull) or stated power?
I'm using a el34 push giving out 28 watts and I hardly go have to go past 9 o'clock on the volume knob. To my ears, the 28 watts seem more than enough for me in terms of power with good frequency extension.

If it's topology, it means harbeths with its low sensitivity are generally incompatible with any SET amp, irregardless of it's power output. Given a hypothetical world where both push pull and SET generate the same power output, does push pull drive harbeths better simply because of topology?

As Almarg pointed out, it is not necessarily efficiency, but the impedance curve of the Harbeths that MAY make them incompatible with most SET amps. Most SET amps have a fairly high output impendance compared to other topologies. Push pull amps can also have a highish output impedance, but, typical SET amps tend to be even higher. The high output impedance of most SET amps will interact with the low impedance of the speaker in ways that will cause frequency response to be uneven. The same can be said of OTL amps which also tend to have higher output impedances. The only way to really find out if such interaction renders the speaker/amp combination unacceptable is to try the amp with the speaker (it is even possible, though less likely, that such interaction will be felicitous).

I am somewhat on the fence as to whether the 28 watts will be adequate or not. Clearly, that amount of power with the Harbeths will not allow for playback at extremely high volume in a large room. But, if you play at reasonable levels, this amount of power may be adequate. As Atmasphere pointed out, long before a SET amp has reached its rated power, it will already be straining. I have personally found that SET amps do distort somewhat gracefully--they become "thick" and slightly muddy and stop getting louder, while pushpull amps will distort more harshly (albeit at higher volume levels for the same rated power). It may well be the case that the higher powered SET amp will be pushed into obvious distortion only for brief moments and you might be willing to live with that. Hence, it is really hard to say one way or another if a particular power level is adequate, which is why some people advocate huge amounts of reserve power (I don't because I like the sound of lower powered amps of all kinds).