The Alpine amps are generally designed to run best at 2.66 to 4 ohms resistance. 2ohm operation is generally not recommended. it puts too much strain on the electronics of the amp, which can theoretically cause overheating and burnout.
2.66 ohms is most commonly used when you have 3 subs - they can be wired in mono to use 2.66 ohms, and they kick some serious thump if you have the wattage behind them.
In a single-sub setup, it would be best to get the single-coil 4-ohm sub and hook it to the amp, bridged mono. The 406 isn't going to knock your socks off as far as power goes, but it should sound good and clean. I use an Alpine T757 pushing two Diamond M5 10's (DVC, at 4 ohms) and it's far louder than I need it to be
