1 large subwoofer or 2 smaller ones...?

I have purchased a few books on audio room acoustics (Sound Reproduction by F. Toole and Master Handbook of Acoustics by Everest and Pohlmann) and both books suggest to have 2 subs in a stereo and HT setup to minimize standing waves. Will also give a break to your main speakers having less bass to handle :-)

But two. One sub always sucks. I use two JL e112 and have tried one for a bit and integration was very bad with one and much better with two.

When is comes to sub bass management, more subs is always better to properly load the room and to reduce modal effects. If it's is worth doing, then it is worth doing right.

Thanks everyone for the advice, and based on the overwhelming preference for 2 subs, I'm going to go that route and add 2 ML Depth i's.

Next question... The Depth i has both line-level and speaker-level inputs. Thoughts on one vs. the other for connecting 2 subs in a 2-channel system?

With two subs you will need to experiment. Run each sub in L and R channel configuration and then try them in dual mono (L+R) configuration. I had best result running my subs in dual mono.