"...also, "speed" is a silly term and I don't think it's applicable to subs or really anything else in audio."
Perhaps in your opinion and in your experience, but not in mine. I have tried various different combinations of three powered subwoofers; 12" and 15" - one 12" I own in a different system and the other two borrowed from friends, with my Omega Super Alnico Monitors, and in all cases I felt the subwoofers could not "keep up" with the single drivers and there was an obvious tonal lag in the portion of the bass delivered by the subs. I and the man who designed/built the Omegas express this phenomenon as "speed", others might call it something else. His recommendation was to use subwoofers either with two 8" drivers in a sealed box or multiple 8" (or at most 10") driver sealed subwoofers. Call it whatever you want.
Perhaps in your opinion and in your experience, but not in mine. I have tried various different combinations of three powered subwoofers; 12" and 15" - one 12" I own in a different system and the other two borrowed from friends, with my Omega Super Alnico Monitors, and in all cases I felt the subwoofers could not "keep up" with the single drivers and there was an obvious tonal lag in the portion of the bass delivered by the subs. I and the man who designed/built the Omegas express this phenomenon as "speed", others might call it something else. His recommendation was to use subwoofers either with two 8" drivers in a sealed box or multiple 8" (or at most 10") driver sealed subwoofers. Call it whatever you want.