1 large subwoofer or 2 smaller ones...?

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

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?

I see that the Depth i can provide high pass filtered outputs to the main power amp, if connected at line-level. So there are three choices: Speaker-level, full-range line-level to both the sub and the main power amp; and line-level to the sub with its high passed outputs going to the main power amp.

Each approach has its pluses and minuses, and which is best will be dependent, among other things, on the quality of the high pass filter circuitry in the sub, and on the degree to which the main speakers may benefit from having deep bass frequencies kept out of them. I would only consider answers that may be provided to be relevant if they are based on experience with similar M-L equipment.

In any event, my main point in responding is to mention that if you are still using the Krell amp listed in your system description (or any other amp having balanced or bridged outputs), and if you go with speaker-level connections to the sub, do not make the common mistake of connecting the negative input terminal of the sub to an output terminal of the amp. The FPB200 has balanced outputs, so both its + and - output terminals have signals on them. If you connect the - speaker-level input of the sub to the - output terminal of the amp you risk shorting the signal on that output of the amp to ground, or at least loading it excessively. Connecting the - input terminal of the sub to a chassis screw on the amp will most likely work ok.

Regards,
-- Al

Has anyone tried using 2 different models in their system. I use a REL Studio III which is no longer made. Could I add a REL S or G model to the system and achieve the same results as having 2 subs of the same model?

I went the 2 sub route and use a Dspeaker Antimode Dual Core 2.0 with mine. For HT I use the Dspeaker 8033s. Either of these units will take care of peakes and nodes in the room.

Good luck,