subwoofer to digital music and movies

First point: Depending on the functionality of your Marantz it most likely sums the LFE signal sent to the Earthquake. Even so, the lest demand you put on room its  correction the better. Getting the subwoofers crossover and gain as close as you can before running room correction will lesson the equalization applied by the Marantz as well as give yourself a baseline of where you like the subs performance against that of the room correction software. The same goes for your speakers. The more evenly spaced and distanced from the listening position the less correction is needed. 

Second point: I believe the Marantz has a single subwoofer or summed LFE output SW1 which usually cuts off at 80Hz. The SW1 output is most likely  summed. I purchased an RCA Y-block to connect both of the subs inputs and heard no difference.

This manual may not match your sub but you should get a general idea of connectivity. Good luck with it and have fun in the deep end.

http://earthquakeeurope.com/Products/Home-Audio/Subwoofers/MKVI-12PIANO/Pdf/EU/MKVI-12PIANO_manual_E...

Thanks for the reply and it makes sense. The sub I have does not have the DSP and is one level lower supernova MKIV - which happens to have the high level input along with the LFE and hence my question on using both. 

Instead should I selling my Earthquake sub for a REL or other brand? 

What is the Earthquake not offering you or what is your goal? 

Why REL?

Earthquake is great. The form factor is big and not pleasing to the eyes in the Family TV room. What I’ve heard of the REL’s is that they are for both music and HT and can be connected to both at the same time. Not sure if Earthquake can.