What is the current draw of your TWO BXI 2006Ds? I used to have some Hifonics amps that were rated for 1500 watts, and they had 120 amps of fusing on each. You are probably trying to pull too much current through that wire.
Ok, I found the 2008 specs for your amps, and I believe they are the same as the 2006 ones you have. The manual says to fuse ONE amp for 200 amps. So, two should be done for 400 amps. It most likely won't draw that much but I think that 2 gauge is not sufficient for what you are doing. Your alt needs to be able to keep that 2400 charged, so you should probably get bigger wire. You might also consider getting rid of the capacitor and getting another battery.