Hey Eric,

More money will many times not buy you better sound, especially if you just move up the food chain of particular manufacturers. Because the amplifier topology usually remains constant, you are most likely paying for increased features and/or more watts. When you consider that you have to double wattage for a 3dB increase (hardly noticeable), I be careful about paying hundreds of dollars more for 15 or 20 addn'l watts. Technically, additional watts will allow more headroom during transients but how much you notice depends on you and your speakers.

You can get better sound with more money but you'll probably need to spend a LOT more and start looking at other manufacturers than the normal Japanese suspects. Arcam, B&K, Rotel, and Anthem to name a few and there are literally hundreds of others. "Difference" is all predicated on how revealing your equipment is and how closely you listen or even care for that matter. For 99% of the population, a good receiver from a reputable manufacturer (such as the 3 you listed) will be more than good enough.

You don't really say what you don't like about your current set-up so it's a little vague. Ultimately, the best sound possible is through separates, imo. You can start by adding a 2,3, 5 or even 7 channel amp to your system as long as your Sony has pre-outs. If you choose an amp that is a good match for your Mirage set, you should notice a dramatic increase in dynamics, better extension, blahblahblah. I'm not a fan of Sony's amps so any change for me would be for the better.

Good luck and have fun