What do these tubes sound like?

I completely disagree with the assessment of the 300B. “Treble is severely rolled off. As are the lower frequencies, which are the very definition of loose.” The performance of a tube at these frequency extremes is a function of amplifier design and speaker interaction much, much more than the tube. Amps using the 300B can and are designed that give a frequency response well beyond the range of human hearing. I recently sold a Wavelength Duetto 300B SET amp that was -3dB at 15 and 45,000 Hertz. Before that I had a VAC 30/30 that was 8Hz-85,000 Hertz -3dB. If this is a severely rolled off top or bottom end then I don‘t understand what those terms mean.

The loose bottom has more to do with the typical low power of SET amps and poor speaker selection, not the fact that a particular amp is using a 300B. Higher power push-pull 300B amps like the VACs have just as much or more bass control as any other amps of comparable power. If you use a low power SET 300B amp with a low efficiency speaker, you may well have a loose bottom and rolled off highs. Matched to an appropriate speaker they will perform as well or better than any tube out there.

They do have a glorious mid-range.

I agree. Of course I have no way of knowing whether the Duetto can actually do 15 Hz because I have Lamhorns with DX3 Lowthers that don't play much below 60. I have them supplemented with an MBL sub that integrates very well and gets me pretty flat down to the mid twenties so I am happy.

I would assume that the speakers you heard the other Wavelength amps hooked up to had similar specs to the Lamhorn. My present amp is a 2A3 Bottlehead parafeed that I am using 300B/2.5 volt Sophia meshplates in. I hope to get some 2A3 meshplates sometime and do the comparison you suggest.

I had the VAC hooked up to some Soliloquy 8.2s that could get into the mid 30 hertz range and they were good at it. I agree that the VAC amps are a unique product and I don't hesitate to recommend them to someone who needs more power than an SET can provide. I think they are in a class by themselves in push-pull amps.