I'm quite happy using a Crown XLS1500. One thing everyone with a Class D amp needs to remember is that these amps are capable of huge peak power output, but most of them will rapidly scale down their available output if fed a high level continuous signal. The Crown DriveCore system, for example, will limit or compress a continuous signal which lasts more than a short period of time. Seriously, though - you can't expect to put 1500 Watts into a Magneplanar for more than an instant without cooking something, so as long as the peaks are unrestricted, the fact that the amp can "only" put out 400-500 Watts for an extended period shouldn't be an issue. For years people have recognized that the continuous power is generally not a concern for home audio; it's the peak power which can be delivered without clipping. My Tympani 1C system is one of the few for which Magnepan made a blanket statement of no maximum power limitation, yet they wisely urge the user to keep 2A fuses in the line (roughly 30-40 continuous Watts) to protect the drivers. They will play LOUD without blowing fuses.
I bridged a Crown to drive a Dayton UM18-22 sub, for example (that's 1500W @ 4Ω), and regularly listened to classical organ music with strong components below 20 Hz. I've never lit the -20dB indicators or the thermal ones, even at wall-rattling volumes. The mid and high frequency panels of a Maggie are somewhat less demanding; I've measured peaks of around 100 Watts at borderline uncomfortable sound levels. I'd hesitate to use an amp with less protection built in, but the Crowns at least should be a good match.