I figured in your sub amp, its part of that 17.5A average draw, lol. Music is dynamic so is ever changing...so does the amperage draw to your amps. A constant 50A draw would be only happen if you are doing spl comps and/or playing a full strength test tone at one frequency. You'll be fine with the stock battery, in your situation a1 farad cap should help.....or not depending on your views of capacitors.
Your car is just like the amps, it won't use a constant 83amps from your alternator unless it's overloaded with accessories and they are all running at the same time. My last car had a 65A alternator and I was running 800W like you.