There isn’t that much difference between 12″- 15″ – 18″ subs from the same brand. My Mackie HD series sub (18″) goes down to 43Hz, it’s 15″ sibling does 44Hz. The big difference is in the measures 3dB max SPL difference of course.
However, it is also this relatively big difference in SPL that causes the “gap” I mentioned, not the frequency range. As they all usually run higher than the cross-over frequency which is (like the 702s) somewhere between 75-120 or so.
Frankly, the smaller diameter subs give a tighter bang, as you said to complement the lows of your full ranges (actually taking over a lot of the energy, giving your full range fuller low-mid and mid), where the bigger ones (18″ and up) give more oomph to the real low end, simple physics, more surface to move that air.
Think you made a good choice. Nice setup.
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