The speaker level input is high impedance, so it won't have a large effect on the Denon.
If you love the sound from your Monitor speakers, why do you need a subwoofer which, as you're finding, isn't just a lug and play- you'll also have to deal with correct placement and other considerations. IMO, a pair of really good or great speakers should be enough to provide very good low freqwuency response without needing to learn to setup and control the DSP and if you look at the specs for your speakers AND the sub, you won't be adding much, but that's not as easy to know because the Monitor specs show the in-room response with -6dB. They need to provide graphs.
The good part- this sub has 0-180 degree phase control, so placement will be easier than if it had a 0/180 degree switch.
Look at the rear of the Denon for impedance info- it's below the binding posts.