Decibels are a ratio: http://www.phys.unsw.edu.au/jw/dB.html

However much you perceive increasing amp power will help, just adding some punch and clarity can improve things. I personally don't think a 545 or 5400 at 100 or 125 watts will be worth the expense for the little gain he would receive.

If one had a 500 watt amp it would be better on the speakers than a 50 watt amp, it's clipping, or driving the amp past it's capability that blows speakers. As long as a speaker receives clean power it can usually handle a surprising amount of power. Because an amp is 500 watts doesn't mean the speaker would ever necessarily receive that much. It's not the size either, my 2.5's use 6 1/2" drivers and they've handled a 250x2 Krell power amp with no problem. The question would be can these Polks handle 250 watt RMS or peak. Either way the same principles I stated above apply.

It's more rare but you can over power a speaker by the cone being pushed out further than it was meant to go. Going past it's designed excursion. So using a true 500 watts on a 250 watt speaker could lead to problems if the amp was turned to full throddle.