There’s been a good bit of mention around the 8c in this thread so I’ll try to chime in a bit.
The 8c does have dual 8" subs on the rear of the cabinet. They do perform best when boundary loading to your front wall or similar surface (I’ve used glass on more than a few occasions). This works best from 4-20". The subs operate from 100hz down. The anechoic response is flat to 35hz, but real-world, in-room response is typically flat to 20hz or lower. At Pacific Audiofest, we were easily flat to 15hz (not -3db@15).
The 8c does have sub outputs. These are full-range "unmanaged" outputs. That is to say that they do respond to volume control but do not have additional filters (crossover, eq, etc.).
In terms of stacking subs, the main benefit of adding subs to the 8c is not additional output at the main position. The 8c is already full range and most any bass issues will be room related. More subs placed in the same position will only exacerbate modal issues. If you have 8Cs and bass problems, look at distributing the bass to resolve room modes, not augment the 8c output.
The 8c does have DSP, but we consider it an acoustic product first, with DSP in place to augment what it’s doing passively. We use DSP to phase align the front facing drivers to the subs, which have a delayed response depending on how close or far they’ve been placed to the front wall. Additionally, the 8c is fully REW integrated, which we use to apply EQ to frequencies below a room’s Schroeder response (typically in the 20-200hz region). Lastly, being an active speaker, dsp is used for our crossovers, gain controls, etc.
Everything else is passive. The cardioid mid range from 100-1250hz is a purely passive design. Our tweeter waveguide allows us to highpass our tweeter at 1250hz to prevent the 8" mid from beaming, gives us a near mirror image on/off axis response. Between the mid and tweeter, we are constant directivity from 100hz up. The rear firing subs, when boundary loading get +6db of free lunch below 100hz, and don’t suffer the typical destructive cancelation of a bass/sub that’s been pulled out into a room. The net result is extremely taut low bass that doesn't strain the bass drivers or electronics. When we apply EQ, we typically remove bass, not add it.
In any event, I hope that this has at least answered a few questions about how the 8c works.
