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Exactly what should I be looking for in a preamp, then?


CV, it isn't the preamp section you should be concerned about. Several sources including Axiom recommend using Denon or Harmon Kardon receivers with M80 speakers as their power supply is beefy enough to run them without going into shutdown protection. Many have reported high enough SPL for their preference as well indicating these units should supply enough, adequate power to run the M80s with reasonable results.
Personally, with a 4 ohm speaker i would buy separates but only to ensure that the speakers had enough dedicated power for a truly high sound output, which sometimes is my preference. Axiom uses Bryston amps at the factory during their testing stages, a very proven name for amplifiers, although also pricey. You can get some older models, say 2 generations back possibly between $1500-2000 Cdn used, depending on the model and specs. At home i'm presently using a less expensive Anthem unit, the MCA30 to power some 6 ohm speakers, but they have been used and are rated for 4 ohm units. You can find the MCA20 amps used for as little as $400 Cdn.
If you go this route, i suggest buying a good but 'lower cost receiver, like a Denon, H/K or Pioneer (see this one here and here)
and use it as the preamp/processor section or to power surround and centre speakers, leaving the amp to power the M80s. Totalling the used unit and new unit cost together is still well under the cost of many higher end receivers.

"Those who preach the myths of audio are ignorant of truth."