I spent some more time looking at measurements and experimenting with parametric EQ, making extensive use of the graph feature in Peace GUI, and the Sundaras now sound much better to me. I'm using these settings:
From the measurements I've seen at Rtings, the Sundara apparently lacks a notch at 10 kHz to bring the soundstage outside of the head. It's easily fixable by EQing in a -10 dB peak at 9,500 Hz with a Q of 25. I like my treble air though, so I used these filters instead:
Peak at 9,500 Hz, +0.5 dB, Q 1.4
Peak at 9,500 Hz, -10 dB, Q 20
Peak at 10,000 Hz, +1.5 dB, Q 3
High shelf at 14,000 Hz, +3.5 dB, 60 dB/Oct
I settled on this to extend the sub-bass to 20 Hz while keeping the lows neutral:
Peak at 20 Hz, +2 dB, Q 0.6
And these filters brought forward female vocals just the right amount for me without sounding shouty:
Peak at 1,700 Hz, +1.5 dB, Q 1
Peak at 2,800 Hz, +1.5 dB, Q 2.5