1 large subwoofer or 2 smaller ones...?

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.

There is a right answer, but unfortunately, not an easy one... it depends on the room. when you add a sub, you add room interaction, you create peaks & dips and have to deal with them. Placement is key. In some rooms it is totally conceivable that one sub would out perform 2 when it comes to smooth accurate bass.
I have 2 subs right now, but use 1... I have moved them around countless times, finally bought a decent mic and started taking measurements, I may be able to get an improvement with 2, but its been a process and have decided to leave it alone.
Good luck, Tim

bass horn, subs are sub optimal and highly compromised designs