There's still some uneven bass response but so far I've only found one song that really shows it off -- Genesis' "Can-Utility and the Coastliners" from Foxtrot.
OK, I found more tracks that showed the uneven bass response
Experimented some more with speaker stand height and found that (a) I could lower the speakers a fair amount without noticeable change in frequency response as long as I toed the speakers in more to compensate, (b) with a 3.5" stand** (2 thicknesses of 2x6) the LF response was very even, to the point that some tracks which I had thought to have uneven bass on the recording (start of Pink Floyd's Meddle, for example) now showed even response.
This height seems to be right on the edge for FR and imaging (any lower is definitely a problem) but going any higher seems to start making the bass response uneven. The speakers are now toed in enough to almost point directly at me, so the visible side of the speaker appears to be less than a half-inch wide.
I did a bit of reading about floor bounce and AFAICS that seems to be essentially a first reflection thing but at low frequencies, ie the difference in path between direct and reflected LF results in some cancellations. The difference in path length seems to affect higher frequencies than the ones where I'm seeing changes, so guessing this has something to do with port and/or woofer height from the floor/wall/ceiling (although a few inches shouldn't make a big difference there either, so it's not clear what is happening).
** I guess if the stand is low enough I'm supposed to call it a plinth, wonder where the transition point is ?
