The Usher 520 is an amazing speaker and is probably my favorite mini monitor of all time. If you are in a smaller room or near field listening, 30-50 watts will be fine. To get the speakers to really open up, I would start at a minimum of 50 watts.
When I was a dealer for Usher, I would run these with 200 watt ASL Hurricane mono bloc tube amps. Lots of fun there.
At your price, I'm almost tempted to pick them up myself.