To answer JBall while we are on the topic of the 5.25” mid-woofer in the M60: as explained above, it is not possible that this component is running out of steam as you turn up your amplifier with the M60s set to large. The more likely scenario is that your amplifier is going into clipping and therefore resetting the M60s to small is alleviating that clipping.
Thanks for addressing this Ian but I respectively disagree that my amp is running out of power. I have a 250wpc Anthem amp and 500wpc Crown amp at my disposal and the results are the same. As I drive the levels to very high SPL I notice the mid range driver moving in and out rapidly and the vocals don't sound as clean as they do at lower levels. Please don't take this the wrong way as I still like the speakers and this only happens when I entertain loud parties. I tried a pair of Genlec speakers at one of my party events and this didn't happen on those but I realize those are pro audio products with different design goals.
Again thanks for the response.