Hi rdwarn,
Ignore the 250-watt power rating of the M60ti's. Buy as much power as you can afford. You can never have too much power--only too little. It's very easy to push a lower-powered receiver into clipping or incipient clipping (where distortion is rapidly rising), which sends a DC signal to the mids and tweeters and burns out the voice coils.
The advice about buying used separates is excellent. I have an old Yamaha power amp, 200 watts per channel into 8 ohms, which is more than 15 years old and is working perfectly. I've used it to drive all kinds of small, medium and large speakers.
As for distortion to listen for, if you hear a sharp "snap" from any of the M60 woofers (very unlikely), that means the voice coil is hitting the magnet structure of the driver and you should race to turn down the volume. In most cases, you hear increasing edginess or harshness as the amp's distortion is rising (odd-order distortion). With a genuine high-power separate amp, your ears will give out before the M60s do.
Regards,
