Class A Solid State Sound

kosst I am not saying that Atma-spheres are the absolute best amp for those speakers but if someone were using a sub woofer and liked the other attributes of Ralph's amps. Why not? Damping factor is not everything. There are those (mostly Mcintosh owners) who say that a damping factor of 50 is more than enough. I certainly do not think that is the case with sub woofers. You can get damping factor way up there by cranking up the negative feedback resulting in a rather harsh sounding amp. Read this  http://www.soundstagenetwork.com/revequip/atmasphere_ma2iii_followup.htm
My point is that although you can draw some good assumptions about how an amp will perform on a given loudspeaker you do not know for sure until you pair them up. I draw assumptions all the time but usually when I have overwhelming evidence. I am a SoundLabs fan. The MA-2 is the pet amp for SoundLabs users. These speakers have a rising impedance in the bass. I have heard figures up to 30 ohms. Perfect for these amps. I would also never have a system without a sub woofer array and I always use a very powerful SS amp with high damping to drive it usually a rather inexpensive commercial AB amp. Using an MA-2 for this would be rather silly from a cost and performance perspective. But, you can see why I have a strong interest in these amps.
Atma-Sphere does push high impedance loudspeakers because his amps work better on them as you suggest and most people can only afford his smaller amps. High impedance does have it's advantages in terms of efficiency. Klipsch Heresys on stands with two sub woofers can put on a surprisingly good performance.  Personally when I see Wilson Loudspeakers the first thing that pops into my head is R2D2. If I were to buy dynamic loudspeakers, which will never happen I would go for a large set of Magicos. 

I have not had a tube amp since 1969. I have blown a couple of SS amps, a Fuzz Linear and a Krell KMA 100. Tubes do have a major advantage in that they are easy to change. Transistors not so easy. I only care about what sounds better. If I have to change a tube once in a while that is no big deal. You settle on speakers and build the rest of the system around them. Since the Krells I have stuck with class A amps.
They have a way of sounding more powerful than they actually are. Perhaps they handle transients better. The Atma-Spheres drew my attention because SoundLabs owners love them and they are for sure my next and probably last loudspeaker. The Atma-Sphere and Pass amps are the only amps in the running. Xs300s VS MA-3s. I seem to be always dreaming about the next system usually without regard to the price tag:)

In term of drive, everything else being equal, BJT usually has more output current output so it should be able to drive better vs. FET.  Therefore BJT sound tends to have more dynamic compared to FET.  

With respect to output impedance, I suspect most amplifier output stage uses push-pull config which is a combination of N and P devices either they be BJT or FET.  BJT usually uses common-emitter config, whereas FET would use common-source, so either FET or BJT they should have fairly low output impedance so driving difficult speaker loads probably shouldn't be a problem.  The rest depends on how many devices is being used in parallel and how feedback is used to control the output impedance.

With respect to the sound, the FET is known to have a more tubey kind of sound whereas BJT has a cleaner, more solid state sound.  I think it may have to do with the FET device being a square device.  That is the FET output current is the square of the input voltage proportionally.  So FET tends to have mostly second order distortion just like tubes hence it having similar sound to tubes.

BJT on the other hands is an exponential device that is the BJT output current is exponentially proportion to the input current.  Therefore the BJT will have a lot more higher order distortion.  Higher distortion usually is associated to the sound being a bit lean, more analytical, and hence being more solid states.

Some amplifier designs use FET as input stage and BJT as output stage just like a hybrid amp that uses tubes as input but the output state is solid state.

In term of drive, everything else being equal, BJT usually has more output current output so it should be able to drive better vs. FET. Therefore BJT sound tends to have more dynamic compared to FET.  

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Cheers George