When choosing the right amp. for your Sub there are a few things to take into consideration first.
- How much power can your sub take?
Most subwoofers advertise their "Peak" or "Max" input power. This is the amount of power your sub is capable of taking, but it is not healthy to run it on this power level for more than 1 minute or so. The power level that you are looking for is your rms (root-mean-square) power rating. This is the level of power that your sub can comfortably take without causing any damage.
With that in mind you want an amplifier to push your sub to its limits, but not so far that it damages it. Try and find an amplifier with an rms similiar to your sub's. If the amp has a higher rms, then just be careful not to turn its output past your subs rms. And if your amps rms is lower than your subs, dont run the amp past its rms for long periods of time as this will cause your amp to blow. For example 1000 watt rms sub with a 1200 watt rms amp - have the amps out put on 1000 watts.
If you are just powering your 1 sub then you wont need an amp with more than 2 channels. If you are powering more than 1 sub or some speakers as well, then get the equivalent number of channels, eg 2 subs- 2 channel amp etc. but be careful to check each channels output as in some subs they vary.