KW, presumably you mean the 7075. Your room size is about average and so is the sensitivity of the M60s. What is also important is how loud an average level you play at and how dynamic the program material is, i.e. how high above the average level the peaks are. For most uses the 7075 should be plenty, and if it wouldn't be it's unlikely that the 7125 would make enough of a difference. The difference in maximum output between 75 watts and 125 watts is only about 2.2dB, so if 75 watts allowed playing a brief peak at say 105dB, 125 watts would allow playing it at 107.2dB, not a very significant difference.
