Myrison,

You can certainly pair the 600 up with another sub. The issue is whether or not the other sub would have the desired SPL level that you seek.

To illustrate, I can pair my Sony active sub to the 600 and my Sony "works". However, its loudness is severly attenuated because the Denon can't produce a voltage signal of sufficient magnitude to drive it due to the 600's low input impedance.

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