I have both the THDI and the OCSD. You're right, AGS is an all or nothing deal. There's some really nice threads that go way into depth on the THDI (search for user Eublet) but whenever AGS is on it acts as a simple cabsim... basically a low pass around 5k with a resonant peak around 1k. The frequency graphs that are floating around very much mirror the OCSD graphs in its manual. The high pass shape changes slightly as you add more gain but not a whole lot. There isn't really a significant high pass in play though, especially at the higher frequencies (30-40 Hz+) that would make a big difference. That being said, it sounds great on its own and I didn't notice a huge difference with the OCSD on or off, so I ran without it. Plus I wanted to fit other pedals (octave+fuzz) for different synthy sounds so the subtle difference with the OCSD lost out to the real estate demands of other pedals.I really liked the "gain" on the Tone hammer. Just sucks that you cant switch it on and off. With it set low it sounded really nice. I bet it thru the OCSD would probably sound nice. But Im not gonna find out! hahaha Im set for now, Im really blessed with the gear I have and I am very grateful.
YMMV of course... I run the THDI with AGS around 3, bass 1ish, treble 11ish, both mid knobs around 10 (all inspired by one of Bethel's bassists). I was surprised by the big mid cut but it sounds good and doesn't seem to get lost in the mix (we use IEMs tho so it's kind of hard for me to gauge that). With my bass tone knob around 50% or so and playing mostly finger style, it's a warm and defined sound with a "soft fuzzy" coating... not overly aggressive. But if I bust out a pick it adds more. And if I keep the pick and add a touch more tone knob (+10% or so) it adds even more (perfect for Lion and the Lamb). For the OCSD, I was running it on 8x10, cab size nearly maxed, high response somewhere 1-2ish, and both mid punch and mid scoop set flat (I figured I did enough to the mids with the THDI).
So if any of your favorite settings on either pedal are different you might have different results and find the OCSD may still make a significant difference. I also haven't experimented with stacking more overdrive pedals with the THDI, so my answer could change as I add in more variables.
With the BDDI, I have the V1, no mid control version... the latest eye opener for that one was how the presence and gain knobs interact. I haven't used it on a Sunday recently but got a nice big sound out of it, which was nice because I was previously frustrated by it and the nasty sound it was making; I guess it was user error... It's different enough from the THDI to justify keeping around but the BDDI definitely benefits from the OCSD being on.. it really adds that 3D aspect to the tone.

