For stage monitors, ultimate fidelity is not really necessary. The purpose of stage monitors is not to hear the mix. The purpose of stage monitors is so that the singers can sing in tune and time with what's in the other mikes, and not what's bouncing off the back wall from the main PA speakers. Spectrally, they don't need the same flat response as playback equipment, and SINADs over 90 (or, probably, 40) are just not necessary. They do need power, because stage monitors need wider dispersion than PA speakers, and therefore tend not to be as efficient.
Rick Wakeman reported that the monitors he used for hearing his keyboards at Yes concerts back in the 70's were Phase Linear 400's--several of them, perhaps one each for each monitor cabinet. But that's perhaps a bit of an extreme use case.
But Crown makes relatively inexpensive and durable amps, in both AB and D topologies, for high-power PA applications, including stage monitors. It says something that their least expensive line are AB amps, and the line above that are D topologies. These amps do not measure superbly for playback, though they are still reasonably competent, but they are good at what they were designed to do.
If I were to use, say, an Ncore-based amp, I would ensure abundant cooling. The full power rating of the amp in continuous service is cooling-based, according to Hypex's documentation. A fan and heatsink wouldn't be out of place in a commercial-sound setting. But even with that consideration, a Hypex NC502MP-based amp provides nearly twice the power of that Phase Linear 400 at 1% distortion, and a little more power at distortion levels much below that Phase Linear amp.
Rick "with a bit of part-time roadie in his background, but
not for situations like Yes concerts" Denney