Low Voltage H.O Alternator??

alright my situation is that i have a 2001 nissan maxima with a stock alternator and i am running 2 batteries 2 caps a mmats 1400.05 amp and a 13w7 jl audio sub. I have a 200 amp solenoid battery isolator under the hood. Yet lately i am running very low on power. My caps wil lread 9.5 at jus an idle and soesnt seem to be chargeing. Also when i leave my car with jus the music playing and off the car will die an not start. Now i knwo the poitn of a second battery is to get aroudn that but in my case it seems to not be working. My question is do i need a new altenator or is something wrong with my 2nd battery or the isolator.

It sounds like your alternator is blown

you have blown half of the diodes in the alternator

you need a new alternator, search, meangreen makes some good ones ive heard.

people need to learn 1st, big 3, then HO alt, then battery...multiple batterie only help if you are planning on playing the system hard w/o the vehicle running or if you plan on compeating and are using so much wattage the alternator cant flow the needed current.
if you have good alternator, good wiring, and a good battery and you are still loosing a substantial ammount of voltage, you have TOO MUCH POWER... to do so you would need many many kW of power or be running a few hundred watts on non solid state or Class A amplifiers...which is downright ignorant (IMO Tubes are good for mids and highs, if you want one on your sub then you arent going to get a lot of power)