You probably can't even hear the difference in volume between 100 watts and 125 watts—it's under 1dB. If you are running out of power, it will happen regardless of the receiver's rating. A good power amp will be stable down to 2 Ohms, have greater power reserves for dynamic peaks and thus will be capable of fully driving a speaker without any dynamic compression.
Ultimately, the majority of receivers on the market will wind up underpowering almost any decent speaker one attaches; including 2-way bookshelf speakers—and most definitely including "reference" towers. Whenever I have demoed other people's systems, invariably it is the receiver's amps that are the weak link. I can hear the amps begging for mercy, and usually it's not hard clipping at all.
Unless you are hard clipping your amps, I doubt you are doing any damage. But headroom translates into quality, and with good speakers, and amp with sufficient dynamic reserves can make a difference at volume levels that are much lower than you might expect.
Edited by imagic - 7/15/13 at 1:59am