"The issue I had was when I plugged in 2 drivers
it didn't seem any different or better..."
one driver, 6.5 cubes, 20hz, grey
one driver, 13 cubes, 20hz, red
two drivers, 13 cubes, 20hz, green
going from grey to red provides a little more
spl at 20hz, but since you are also likely to
get some room gain down there anyways, i'm not
sure that is much of an advantage.
going from grey to green provides 3db more
sensitivity, so for the same power input, the
dual sub setup will be louder.
on the "max spl" chart, going from grey to red,
provides little to nothing, but going to the
green (dual driver) setup yields 6db more max spl.
so with a larger cabinet and a single driver,
there is a marginal advantage right around tuning,
but that is all.
going to a dual driver system yields 3db more spl
sensitivity and 6db more max spl.