Your car amp adjusts volts/amps to the required wattage/ohm needed.
Your amp can not make power appear out of thin air only tweak voltage or amps thus it can only output what it takes in.
Hence it all comes down to how many RMS watts your amp has (or at least what your speakers require) compaired to the watts of your 12 volt rail which can be calculated as 12 (volts) * amps of that specific 12v rail.
Now if this is not a top quality PSU then there is zero garantee what the real amperage is of that 12v rail.
Hi there, the pc psu is a solution for a headunit or some other low power stuff like eq's or crossovers.
Don't use such a setup for your amps, i've tried it all, i'm a former car audio shop owner so i have several caraudio amps.
It will actually work, BUT the output from pc psu is 12v with little tolerance like +/- 5%, also a psu doesn't really like to deliver real power, and it definitely doesn't like the transients in music, a car amp can go from 4-5 amp in a silent passage to full blown 70-80 amps when the bass drops.
The output from your car's alternator is somewhere between 13.6 and 14.4v (with an average around 100 amps) while some amplifiers will accept 15 to 16v, especially oldschool amps like phoenix gold M and MS have unregulated power supplies, wich means the higher the input voltage the higher the output power will be.
Next problem with the psu is that it has a ripple, the better soundquality your amp has, the more you will hear that ripple thru the speakers.
The only good solution to do this is with a battery and a good quality charger.
It's not the cheapest solution but it's definitely the best and most reliable solution.
I have a Phoenix Gold MS2125 in my living room on a set home speakers, connected to a optima red top with a 16 amp charger and a 1farad capacitor, this setup blows away most home amplifiers both in quality and power, plus it's just cool to have a Gold plated circuit board in your living room.. ;-)