Sound quality.
A pro amp is built for stability under varying loads (a rental amp might run a variety of speakers in a given month), hardiness in travel (for rental or band use, etc) and, typically, doesn't have to be very quiet, given the volumes a live show is likely to produce (The amps noise floor can be high and its use of cooling fans).
An audiophile amp will have a much lower noise floor, greater frequency range and wider dynamic headroom (can play relaively softer and louder without tweaking the gain control). It will also sound scads better; the better your speakers, the more likely you are to recognize this.
The Magnepans deserve a good amp. While you may not be able to afford Krell, take a look at
these. The Adcom GFA-555 is 200WPC, stable to two ohms and can sound absolutely amazing for what you pay for it. In fact, leading it with a tube or tube hybrid preamp, like a Moscode Minuet (which is what I originally wanted) or VanAlstine SuperPas 3i (which is what I have) will warm up the sound of the 555 quite a bit. A good team for your Mags.
Not enough power you say? Well the GFA-555 is bridgeable. Put two of them in Mono configuration and get 500 Watts each.
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