I had a pair of whatever the Marchiotto DQs 3 ways were with an 8 inch woofer and a grill that was entirely sock clad -- I don't think they were 20s, but can't identify them, to my chagrin. Beat the daylights out of my regnar modded DQ10s. So much that I bought them back from the guy I sold them to. But then sold them again. I have the same response to the Alon speakers I've heard -- I love them, but there is something going on in the low treble that gets me after a while. It may be relevant that I use Dynaco Stereo 70s -- both stock and Van Alstine modded -- which ought not to have enough power. But I never listen very loud, and I finally just decided to stick with the Stereo 70s after a long flirtation with the Parasound/John Curl 250 wpc amp (model number forgotten), when I realised that the DQ 10s were more dynamic and sounded better on the Stereo 70.
So I vote for the later Dahlquists. But I am not sure I vote for $600. There is an awful lot of really serious competition out there for that kind of money. Vandersteens, for example. Or Lowthers, depending on your taste (and this is a serious suggestion -- the Dahlquists (like most speakers, they are not really full range, so if you need low bass, you are looking at a subwoofer, so suddenly you are at a noticeably higher price range, whereas, if what you are looking for is phase coherency, single driver designs might give it to you with less penalty from crossover loss).
But if you like the DQ 20s a lot, I imagine other speakers will not satisfy, so see if you can get to a price you like.