why is my system not loud enough?

So you have 750wrms of subs and you're putting out around 1900wrms?

With an 80A fuse at 14.4vdc and 90% amp efficiency you are sitting right around 1036 watts.  This is assuming you are pulling 80A through the fuse and your battery voltage is steady at 14.4 (both of which probably isn't true) and that you have a very efficient amp (which probably isn't the case). 

My guess is the amp is probably putting out closer to 800-900 wrms or so but that is just a guess. 

With that kind of guesstimated current draw your 6awg actually should be sufficient for what you are doing. 

Like I said before, if it were my car I would ditch the 12, buy another 4 ohm 10" sub and build a custom box to house all three subs.  If you want more output (SPL) I would port the box (if those subs respond well to a ported box).  If you want a deeper, cleaner bass a sealed box would be the preferred route.