Way to go, Adrian!
I think I found my Pout problem... me. I finally got serious about the o-scope measurements, verified the cal of the vertical amp, dug out the ground clip for the 10x probe, and measured 40Vpk from drain to source of Q1/2/3 with almost no ringing. Vp-p on the BNC connector was 32V.
Or it could have been something loose when the QCX fell off the bench.
One more try with the LPF response...