Best modern speaker to match old Mark Levinson

Timlub, my response was a bit tongue-in-cheek and was intended to infer that he can pretty much use any speaker he likes without reservation to drive capability. Sound quality is a different matter.

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

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

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.

Audition the Thiel CS 3.7.

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!!