You have the 600 to give you linear lows so you don't need HP drivers for down there. However, the HP drivers would be very handy for very linear 55 Hz to 85 Hz response which is where the mid-bass lives.

The larger cabinet allows the 60s to reach 14Hz lower than the 50s.

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