100W RMS for these speakers would be just fine. It is better to overpower speakers and set the gains on the amp then underpower them.
I am running a 5 channel Audison amp (LRX5.1K). It gives out 160W x 2 to the fronts, 50W x 2 to the rears, and 750W x 1 to the sub all into 4 ohms. I am from Canada so I can't compare it in price to the 500/5, but it is great bang for your buck. If you wanted to save some cash you could get the LRX3.1K and skip the rear fill (it has the same specs as the 5.1K but without the 50W x 2).
I think this amp combined with the XRs and a 12" sub would be a nice little SQ system.