If you only want to power up for some measurements except full load test, then a 4 A 12 V supply might do. Big amps tend to have quite a nice quiescent current, I observed. PC power supplies can't do a lot af amps at 12 V , but they did it at least sometimes for me. Watch for your pc-power supplies needs. As almost all switching designs, they need a minimum load. I went good with a 12V / 50W bulb over the 5V rail when drawing maximum power out of 12V. Maybe your supplies are better and don't request such loading, but some do. ATX p/s are AFAIK switched on by pulling the PS-ON pin to GND.
You can, regarding the available output current of your power source, not expect maximum output of your amp.
Also watch for high curents when connecting your amp. It might fill its internal caps with quite some amps during the first few seconds. Many workbench and also pc power supplies trip out, then. Simply reset the overcurrent trip quickly several times.