Everyone who knows me knows that I have been drinking the Zu Kool-Aid for years now. I agree with Phil (213Cobra) that finding the right amp is fundamental to building your system around the Definitions. Having patience and willingness to try numerous topologies will give you the best chance to find the best match for you.If you are buying the Definitions or planning on it, my suggestion is to plan to experiment to find what pushes your buttons. "one and done" is a very unlikely approach. We all value certain playback features differently and that shows with the great variety of amps Definition owners use. Try every amp you can on the Definitions. The great part about them is that just about every amp on the planet will drive the Zu's to satisfactory levels. I believe I have heard every topology on the Defs and I can see why some would want OTL's, 845 SET, monster SS, chip amps etc...They all have strengths and weaknesses. Nobody can choose for you. Be adventurous (and report your findings back).
That being said, there is one area that most Definition owners neglect and, to me, the first component to address (even before trying to find a matching amp): the speaker foundation. Replacing the stock spikes with a proper foundation unleashes a whole new level of performance that will be appreciated from the very first note. There are 4 Definition owners in Atlanta and we all own the same speaker stand...Obviously not a coincidence. I feel confident speaking for all of us that it was money very well spent. For me, it may be the biggest performance upgrade of everything I have tried(which is a looong list). Sounds ridiculous...I know.