Paul,
I agree that, in terms of definition, parallel SET is an SET. But, the original poster is asking about what, if any sonic differences there are between use of one and more than one output tube. You are one of the most experienced poster so I, too, am interested in what you have heard.
Mike,
I suspect that choice of approach involves trading off compromises. PSET offers more power than using a single tube, but, it is more complex and difficult to implement. In the SET world, there is a bit of a "religion" when it comes to simpler is better. On the plus side for PSET, is the lower output impedance, but that advantage would probably require that the output transformer be designed to take advantage of the lower impedance.
I have been told that matching of tubes is extremely critical with PSET, particularly because of the problem of one tube ending up doing most of the work when an imbalance exists. In my amp, I've run the supposedly delicate EML meshplate 2a3s for a very long time. The tubes still sound good, though they now measure weak and quite grossly unbalanced in terms of transconductance (measured on an Amplitrex tube tester which tests at full power). Then again, I don't really push my amps very hard when playing them. So, I don't really know if this "issue" with PSET is really true, or if Audionote has a better implementation than some other designers, or I just don't really need the extra power of the PSET approach. All I know is that I heard, and preferred this amp over any other competitors.
