Old power supplies

After losing my pc speaker setup I started to fiddle around with three old psu and a car amplifier. I know this sounds like a perfect receipt for a disaster but I'm fully aware the dangers. In my possession I have a 300w 250w and a huge 150w I took out of a p3. The amplifier needs a massive 40 amps of 12v to fully brings out the beast but I'm only using two of the four channels and I don't love making noise specially when gaming. The smaller 300w and 250w have both 10 amps on the 12v doesn seems to compare to the 150w with 4 amps 12v in terms of clipping. The huge 150w provides a steady stream of power at high sound volumes which the two higher rated psu cannot do they produce sound clipping to quick. Why is the 150w with the bigger caps better then the smaller 300w and 250w ?