Biggles on this account, couldn't be bothered changing.
You shouldn't need a battery upgrade, as you are not running large amounts of power. If its all in and your headlights are dimming, etc then you'll have to look at those kinda upgrades, but normally you'll be fine.
XM-504Z is your amp. I'd say leave this to speaker duty and get a response monoblock for your sub. I say this because the $ per watt your paying can't really be beaten. Although matching your sub and amp isn't hugely important, there is still a reason they are rated. You will notice a 400wrms amp compared to 100wrms (which is close to what your giving it now). You will get more volume, clearer and drive the amp less which is better for it in the long run, and your giving the voil coils what they are after.
As for the RCA statement above, its true. Give it a try.