well in any case.... here's some usefull info on ATX power supplies....

ATX power supplies differ from AT power supplies in several important regards. Major among these is that ATX supplies depend on logic circuits on the motherboard to turn it on. Once the power switch is flipped, the power switch on the ATX power supply (some ATX power supplies do not even have such a switch), the ATX power supply sends a 5V 720MA current to the motherboard through pin 9 on the power connector. That current is for WOL (Wake-up On Lan) and power on circuits. A power-on jumper on the motherboard connects to the pushbutton located in front of ATX case. When the pushbutton is pressed, it sends a signal to the motherboard, which in turn notifies the ATX power supply to turn on full power. The location of the power-on jumper on the motherboard is manufacture dependent. One needs to read the motherboard manual to locate that jumper. At any event, do not try to manually jump-start the power supply without attaching the motherboard. The power supply is expecting certain sensing circuit feedback to regulate the output voltage, manually starting it without attaching it to the motherboard could cause damage to the power supply.

While we're at it, you can power up an ATX power supply by shorting the green wire with any black ground wire. As you know, the ATX power supply will not turn on normally without a motherboard connected to it. There are times when you may want to test an ATX power supply or use the voltage from one but don't really want to hook up a motherboard. There's only one green wire and there are plenty of ground wires so it's easy to do. To use a bent paper clip and insert one end in the green wire connector and bend it around and put it into a black wire connector.

nice read with detailed instructions.

How to Convert a Computer ATX Power Supply to a Lab Power Supply - WikiHow

If you are just going to do some basic power testing (turn on, and test at low output), you can probably get by with a computer supply. A better idea however, would be to get a proper 12v power supply so that you have a bit more leeway with what you can do. I have an Iota 55 amp power supply that I got off of eBay for a great price, and I have been very happy with it.

I do this setup all the time. Red is not the +12v wire. Its usually the Yellow one. To be safe since colour codes may differ, use a Volt meter to test the proper voltage.
Its nice to have a power supply with the 0 | 1 switch on the back. That way if you overload the powersupply, you simply switch it off and on and it resets the breaker.

Most of the power supplies i run display only 15-25 Amps.

This is a cheap solution, but the power suppplies on computers are "UNREGULATED". The expensive $200 test lab supplies you buy are regulated.
The difference is that when a heavy load is placed on the unregulated, it will drop the voltage down significantly therefore increasing the current and leads to overheating. The regulated power supply will maintain its steady voltage within a very close amount.

Don't expect to run over 300-400 full wattage out of your amplifer.
Try to obtain old SERVER power supplies from ebay. Some of those delivery incredible power. e.g. 40-60 amps @ 12 volts