MINe 109 said:

Big, serious Audio Question: can a 20wpc tube amp drive a pair of Quads?

Which Quads?
The original ESL ( 57 ) might do OK, I doubt if the 989 would come to
life with your (Scott or Fischer?) amp.
Also, remember that the 15 watts Quad II amps used heavy feedback
around the output transformer (cathode windings) , thereby being able
to deliver some current.
Your average UL or pentode tube amp could get in trouble.

I'm doing okay with 40 watts of solid state, so, maybe.

The 40 watts figure in itself is quite meaningless unless the amp is
able to maintain the same voltage in lower phase-shifting impedances.
My PP KT88-in-triode amps are able to create a very nice sound quality
in my Magnepans, but my hybrid MOSFET amp goes beyond that in terms of
subjective loudness and bass extension (punch).
I *know* that Maggies aren't Quads, but in my opinion Quads are a bit
harder to drive with mere UL or pentode amps.
Maybe PPP KT88 or KT90 in triode is the way to go, that would give you
about 40 watts and enough damping to control the lower end.

Maybe mr. Devil could enlighten us on what kind/configuration of
amplifiers he uses?

--
Sander deWaal
Vacuum Audio Consultancy