So I just spent three minutes measuring. I threw on a 50Hz tone, cranked my Denon's s LFE output to max (+12) and cranked the Denon's main gain.

With the 600 disconnected, I was measuring 5V rms on the cable. With the 600 connected, I measured 1.5V rms (not good). Doing the math, the 600's input impedance is about 43% of the Denon's output impedance. Again, not good. To prevent significant loading, the 600's impedance should be at least 10 times larger than the Denon's.

So, the 600 is loading the Denon's output too much. I suppose one solution to daisy-chaining is to crank the sub gain on the receiver and turn down the gain on the back of the 600. Give that a try.

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