the size of the amp depends on the size of the subwoofers (amps not diameter). I have 2 1200 Watt subs so I got the 1200 watt amp to power it. I could single power them with a 1200 watt amp each but thats too much. Get the size amp that matches the subs to get most of the power out of them.
If you underpower the subs with a weak amp, you might as well save money and max power weaker subs to get the same sound quality.