I can tell you right now that even with my 600 unplugged, I can't daisy-chain another sub with the 600 without losing a considerable amount of gain from that sub. The amp has a high input impedance.
Will there be a difference in effect between daisy chaining and putting a Y-splitter right at the Denon input and running a separate Coax to each sub?
I've got my second sub on order, so this is an informative discussion for me in determining how I'll be doing my own setup.