I find that when I cross over at 80, the bass is too "fat"
and, dare I say, "slow". It also distracts from the fine details of the music and doesn't provide the right "punch".

Also, when I use LFE+mains, the bass is over-powering. I tried a 60Hz setting like John suggested but I still find it too "fat". 40Hz sounds just right for music and movies. So I paid roughly $100/Hz for the 600 and about $0.07/Hz for the M80s. The 80s are a steal \:\) .

Of course all of this is configuration-dependent so every one should tweak until it sounds just right for them.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated