I'd read the lithium battery post that is around here. After the info presented in there, I'm a little iffy on Xs powers lithium batts.
Edit. Not just Xs btw I'm iffy on all lithium batts. Maybe AGMs or a cap bank? Just be sure you know what you might be getting into before you spend so much money. i can't count the number of times I've made some pretty stupid car audio decisions/purchases before I knew everything about the product I wanted.