It's said that humans can run up to 30 mph. But, for how long could they do that?
Actually, they say the average is about 11 mph but, again, how long could you survive doing that? And, do you need to drive a car 150 mph in order to get good performance from tires rated for 150 mph?
The only reason I would add a power amp is to get the speakers to their full potential, and to push them beyond half volume without distortion, I cant track down the exact specs but I believe they were capable of handling over 500W RMS per channel.
Just a speaker says they can "handle" 500 watts, I wouldn't look to feed it that much for too long. Even though it might survive for a second or so, there's no guarantee it'll sound good while doing so. It sounds like you're finding that out already when you say you can't push the volume over half way. The laws of physics dictate that, without expensive tricks, small speakers can only move so much air and, therefore, only play so loud.
On the average, you use about 2 watts at average/loud listening levels. With 20 decibel peaks, it may draw 20 times that but that would put you at painful listening levels.
So, I wouldn't be too quick to go looking for 500 watt amps. One of the receivers from the brands I mentioned in your price range will drive your speakers to their maximum audible advantage.
[edit] I reread your post and saw your mains were Mirage 1, not 1s. Go for a big amp, but make sure it's stable at four ohms. With their sensitivity rating of 83 db and noninal 6 ohm rating, 4 ohhm minimum, it sucked that poor AVR dry. As for the other speakers, well, everthing else still stands.