Most modern 21" drivers are designed very similar to 18" drivers, except the larger surface area (Sd). That extra Sd means it needs less excursion to reach a similar sensitivity as most 18" drivers. With the same Xmax as an 18" driver, that means there is (at the same power level) unused volume displacement (Vd = Sd x Xmax). The potential is there to do more than the 18" but it needs extra incentive, like EQ around 35 Hz or something to boost the low end.
If you're a newbie I would recommend using the 18DS115 or the 18NTLW equivalent, EQ can fcuk things up real quick and might be counter-intuitive.
Also I would use Cubo Kick 15 instead of Cubo 15, Cubo Kick 15 is solely meant for 80 - 200 Hz (for the most kick impact), whereas Cubo 15 is a 40 - 100 Hz bass bin.
