For smoothness and low distortion, it's hard to beat electrostatics. You might consider the Janszen Carmelita passive. Janszen's current lineup are unusual (if not unique) among electrostatic speakers in not being dipoles.
As you might expect, the speakers are somewhat directional in the high frequencies. This leads to a somewhat smaller sweet spot (though not head-in-a-vice), and also means greatly reduced interaction with the room, so less trouble with reflections from sidewalls and ceiling. Bass is handled by cone woofers.
I use the bigger model, the Valentina passive, with Apollon amps built around the PuriFi modules -- if I'm not mistaken, the same technology used in your NAD M33. I find the combination to be excellent, the best I've had in my many years of owning audio systems.