either way buying another xfl (d2) you'd be spending the same for another amplifier. ultimately up to you what you do but if it was me, ditch the whole 4 guage idea run 0/1 everywhere, big 3 In ofc 0/1, front to back run of cca or ofc 0/1 sell the battery you have now, upgrade to the largest you can afford for the rear and up front do some research even the beefy durlast group 31s from sams club would fit the bill. buy a second of your amplifier strap to the sub you have now and worry about alternator upgrades. just be smart with your gain follow the many proved methods of matching the output from you headunit for clean power. my way instead of having to sell your amp and sub and possibly your box you stick to what you have now and just build up from there.