honda civic 2001 battery and volts.

[desk0]

so, i have the d51 battery in my civic but when i turn the volume up the volts drops to a 11v playing music with bass.

my system consist in

one rockford t1000.1-bdcp for the subwoofer (dc audio lvl 4 12")

and one rockford t400-4

HU Alpine cde-147bt

clarion eqs-746 i think i forgot the model.

and stock ALT

so i was wondering if i replace my battery with a 34 size keeping the stock ALT, would i still have the same problem with volts droping that much, sometimes it goes down to 10v and there is when i turn the volume or the bass level down.

i would consider the fact of replacing the stock ALT but i read that it wouldn't help since it has the ELD.