a dual 4 ohm sub can be wired to have a final impedance of 2 or 8 ohms
a dual 2 ohm sub can be wired to have a final impedance of 1 or 4 ohms
the pdx 1.600 does 600 watts at 2 or 4 ohms
id get the dual 2 ohm cause if your pdx 1.600 ever dies and you want a new amp, you can get more wattage for your dollar, being able to run your sub at 1 ohm., since most amps put out more power, the lower the impedance
example, an amp might have these specs
$250
1x100 at 4 ohms
1x200 at 2 ohms
1x400 at 1 ohm
if you have a 2 ohm sub, it gets 200 watts. but if its a 1 ohm sub, it gets 400 watts