I've owned the Maxx 2's for two years. Based on my own experiences and comparisons I would seek audition of the Audio Research 210 or 610T mono's, Atmasphere MA2mk.2.3's and Lamm Hybrid models. I've also heard very positive feedback from freinds with the Maxx 2's who have tried the VTL Siegfrieds and S400.
I believe Wilson uses a pair of the Audio Research 610T's as references, as well as some of the Lamm amps.
I have owned Essence amps/preamps and _cannot recommend_ them for a variety of reasons based on my experiences. I'm surprised they were even mentioned. If you own the ML 33's, then you own exceptional solid state amps that will not sound "flat" dynamically. No need to go looking for headaches.