Good input. I dont expect smaller woofers to equal larger woofers for performance, at least not exactly, especially in bass. Clayton's published sensitivity numbers already reflect that.
Besides, I have multiple SVS subwoofers for bass. I would use them whether I purchased M3/4/5/6 Sapphires.
However my questions were more about how does the character of the M4/6 sound with their smaller woofers, compared to the 15" models? Since the woofers are handling everything below the tweeter (...586 hz IIRC?), there is a substantial role they play in mids and upper bass that could in theory affect how they sound. Just looking for experiences... if anyone has heard them yet.