Ahh...for powered subs and a huge amp. I thought you just wanted your audio system to run longer by battery alone.
duh... You need to speak with someone who installs them for a living. ( because I hate them)
You're still taxing the system and you need to start with a higher output source. ( alternator)...and work from there.
One more bit of advice. Dont pull up next to me and turn it up. I WILL draw my 45 down on you...
( just kidding...it's against the law here to make that much noise with a car audio system ...thankfully..)
Now if you were talking guitar amps....that's a different story..