Two possibilities:

1. The subs are loading down your Denon. Unplugging a sub relieves the load.

2. The phase relationship between your 600 and the other subs is causing a dip when the 600 is turned on and a rise when it is off.

Regarding the first point, I cannot daisy-chain a sub to my 600 as my Denon gets over-loaded and output is reduced. I've tried daisy-chaining my Sony with a Hsu and have had no problems. But if I try that same Sony with the 600, I end up having loading problems.

Although I am not thrilled with this, I doubt that I will ever use multiple subs so I am just living with it.

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