As you can see these days, amplifiers are being rated with 13.3 +++ power voltage. That means that you need to at least maintain that power supply to the amp to get to to produce the power ratings of it. With older amps they were always / mostly underated power spec. and were rated at 12 volts. This means that at 12 volts you will get what the rating was. If you provided the amp more voltage then more power (wattage) can be produced.

Stick with 80's and 90 amplifiers up to 1996 and you will be fine. about 80% of amplifiers were rated this way yet not always built the same.

So don't go buy the latest and greatest as the new stuff is not as good as the older stuff.

Remember that saying " They just don't make them like they used to". That applies to Car audio amplifiers.

Good luck and buy old amps like the following. Blade, Hifonics, Zapco, Rodek, Orion, PPI, Rockford Fosgate, Crunch, Autotek, Lancer, Denon, Alpine, and of course some others.