When you say bottom 2 drivers, you mean the 2 woofers on one speaker or the bottom woofer on both the L and R speakers?
I will need to recheck but it seamed the noise was only coming from the right channel. If it's a problem of the M80s not being able to reproduce these frequencies, shouldn't it come from both speakers? I guess a good way to find out is to swap the L&R channels.

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."