the bottom line would seem to be that when playing recordings having
the kind of dynamic range you described (about 32 db) you are not
hearing the Franks at their best.
^^ This.
If you really want to hear what any SET does, to really do them justice, the speaker should be of efficiency such that about 20% of full power is *never* exceeded. With a speaker of only 92dB (and while I'm not a fan of Stereophile, in this regard JA's measurements can usually be trusted), that isn't going to happen unless your listening environment is very restricted.
Put another way, to really take advantage of most 300b SETs, speakers with +100dB is really the only way to do it in an average room in the US.
It is for this reason that out of the list presented initially I chose the Nagra.
But if you want to get more in depth about it, there isn't anything special or magic about the 300b. Its a good tube, but often SET advocates prefer the 2A3. That too is a good tube, but often SET advocates prefer the type 45. So 25 years or so ago, the 300b was the one that had the inside track, by 2003 or so the 2A3 was King, and now its the type 45 and its brethren (sub-1 watt-output). What's happening here is not so much the tube, but the capabilities of the output transformer combined with that tube. Usually the plate current of the power tube is taken through the primary of the output transformer. The transformer can take a bit of DC, which causes a DC magnetic field (in a push pull transformer, the two aspects of current flow cause a DC cancellation so only an AC field exists). To minimize some of the effects of this (since the DC field leads to saturation of the transformer core), the core has a saw cut in it to vastly reduce the DC aspect. This however makes it less efficient for AC as well. So this leads to two practical aspects: bandwidth and distortion caused by saturation of the core.
In both cases, by simply using a tube that is lower power, this problem is reduced. Practically speaking, the upper limit of power on this account has been traditionally about 7 watts; above that the bandwidth and saturation issues really close in around the designer (not saying they can't be solved, but it gets prodigiously more expensive to turn out a transformer that actually has respectable 'hifi' bandwidth)! That's why the 2A3 sounds better and so on; get away from SET operation and you get away from this problem.
I've run plenty of SETs in my day. They all jive with what I mentioned above- a type 45 amp so far is the best I've heard. But it made so little power (0.75 watts) that for the most part it was impractical. I built a pair of type 45 amps that are push-pull; on the same speakers (which were 100dB; not nearly efficient enough for a 45 SET) they were able to reveal much more clarity in vocals and instrument details- bringing just a greater sense of musicality. The push pull version of the 45 makes about 6-7 watts.
In case it isn't clear at this point, SETs need efficient speakers! When you push them harder, the distortion not only obscures detail but it makes the amp seem 'loud' (hence their 'dynamic' quality so often mentioned). IME, in about 95% of audiophile conversation using the word 'dynamics' you can safely substitute the word 'distortion' without changing the meaning of the conversation, other than to make it more truthful.
By comparison, any push pull tube amp has a greater amount of usable power- instead of 20% or so you get about 90% or so; even though you are playing higher sound pressures, it doen't sound as 'loud'.
