I know that the 150 and the QS8s get anemic below 75Hz but I've found that this is what works best. Otherwise the 600 sounds too darned "bloated" and "thick" with its 4th order, large vented enclosure. I find that I don't miss anything out of the 150 and the QS8s with a 40Hz cross-over.

BTW, this is what I liked about the 350V3. No "bloat", "thickness" and better transient response ("tightness"?).

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