Why is my system drawing so much current and producing less power?

I have a 75 ford. It has a 250 amp alt. I have a 1200 watt pioneer d9601 and 2 of the 1000 watt rms dual 4 ohm subs. They are wired to 1 ohm. Eherything had worked great for about a year but now the volts drop to the low 11s with deep bass. The system seems weaker as well. I checked all connections. I replaced the alt with a new 250 amp alt I ran new 0 gauge power wide, re did the big 3, added a second 0 gauge ground wire to everything and no change. I think the issue has to either be the subs or the amp.

Your battery probably needs replacement. The continuous charging required when playing a powerful system loudly eventually wears them out. Standard automotive batteries are not designed for it. The best fix for that problem is to install a dual battery system with the amplifier wired to a deep-cycle battery that is dedicated to just the sound system. Otherwise, plan on replacing your battery at least once a year.

Or the battery.

I suggest a hearing test.