That's where a bit of research is involved. I find the speakers I want to use including subs. Then I look for and amp that will power them at or close to what they need..Paying attention to brands and lines as they vary in distortion and reliability..If you want to keep it simple and not have all kinds of extra equipment in your car you may want a continuously variable crossover built in, or the ability to bridge your amp and how low you can run the impedance without hurting your amp. Phoenix Gold for instance is a fairly clean amp one of the best , but some of them put out tons more power and can be used to get loud. Or you can look at Earthquakes PHD2 which isn't particularly clean in fact has lots of distortion but dollar per watt is **** cheap and can be used to compete in DB Drags and is by several competitors.. Reading info from different forums such as SoundDomain or carsound.com can help you out a lot as far as who uses which amp for what application..Another is to read one of the many car audio publications like CA&E or AutoSound 2000 for the amp reviews and such. They are very thorough and give power ratings distortion levels and all sorts of other useful info.
Your friendly neighborhood subaru freak!