Ignore peak or max power ratings. If you're not looking to use an amp for your speakers, any aftermarket speakers will handle the small power the head unit will provide. Match the sub rms power and ohm ratings to the amp's. A little off is fine. A 2 ohm sub rated at 250w rms will need an amp that doee about the same. Dual voice coils (dvc) are a little different, a dual 2 ohm will need the power at either 1 or 4 ohms. A good site to order from is Crutchfield, if you order fron somewhere else be sure to check with the manufacturer to ensure you're ordering from an authorized dealer. Post here with some ideas of what you're looking at and we'll offer advice and opinions. Also, depending on the sub and amp powering it you may want to consider powered speakers else they could be drowned out by the bass.