Think I figured it out... I had previously been listening to the M3s on stands, so when I switched back to M60s I just set them up outboard of the M3s and didn't move the M3+stands in at all. The result was that the M60s were too far apart for seamless soundstage but I hadn't listened enough to them in that position to realize.

When I lifted the M60s the soundstage went from "hole in the middle" to "almost but not quite seamless". I moved them in a few inches on each side and the "hollowness" seems to have gone away.

It's always hard to be sure without going back and forth a few times, but short of building a hydraulic switch box I'm not going to have time to do that much testing for a while.

Anyways, very happy with the M60s now, more so than at any time since I got them.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8