Ok. So I am assuming that you had only the 60s on when you read 75dB. So according to my very rough calculations, you are putting in less than a watt into each 60. If you had all your speakers going, you had far less than a watt going into each speaker. Now granted, you may have cancellations due to your room geometry and maybe it's not a watt, maybe it's two or three. But I doubt it because I used the anechoic sensitivity for the M60s (89 dB rather than the in-room sensitivity of 93dB).

So the power level certainly doesn't begin to explain why you are hearing a difference. We are again no further ahead on this puzzle .