Ive owned the 590axII as well as the 509x - I like the 590axII in regard to sound quality. The 509x is a hair more anylitical sounding? I don’t want to say dry - BUT I've had 3 590axII’s and they ALL buzzed - they all had audible transformer buzz, a mechanical buzz from the units and THAT drove me bonkers. The 509x is dead silent. They’re both gorgeous looking but if you’re in the 10k range for 2k more the Pass labs INT-250 is much better than either one... By a landslide. Not the 60 not the 25 (Ive owned them all) the INT-250 is a spiritual awakening.
Speakers I had with these units: Heresy 4, Forte 4, KLH Model 5 and Wolf Von Langa SONS. They all sounded really good with each amp BUT the forte IVs have something special about them.