Best modern speaker to match old Mark Levinson

Look for a speaker that requires high current, has large impedance swings and is deemed difficult to drive.

You were smart to hang onto the 23.6 and 26S. As a former ML 20.6/26S owner, I wish I kept mine!!

Audition the Thiel CS 3.7.

The easiest advice is to find ANY speaker that you like. The ML 23.5 will drive anything short of a dead short. The amp had the then ML sound which was just a shade dark but overall one of the classic amps ever made. I have never heard a speaker system sound bad because of a 23.5 amp. I liked the Quad 63's and Wilson voiced many of his speakers with ML amps. The newer ML amps I have heard are not appreciably better than yours.

I would recommend Revel Speakers, either the Studio2 or Salon2.

Those amps will drive just about anything, your are lucky that you can explore the universe of speakers and pick with confidence that your amp can drive it.... I don't think though that I'd recommend finding the hardest to drive loads that you can find just because your amp will drive it.
Your amps are a bit on the dark side and lack a touch of air, but overall are very nice sounding machines. I would recommend looking for something with a lively presentation. That should mate quite well with the ML House sound.
Tim