Having just done this last week, here is my .02....buy a set of decent component speakers, the best u can afford...Focal, Soundstream, JL audio are a few great brands ...mount the tweeters in the fairing in place of 2 gauges(u pick) and yes , DEFINITELY use an amp. The sound difference and quality is night and day with the copmponents and an amp.
Don't, I repeat do not buy a cheap set of E-bay speaker adapters, use the Hawg Wired adapters as they bolt in perfectly in 5 minutes.
Solder ALL your connections, even though you will be tempted to use quick connects, don't do it.
You can get a good set of speakers , amp, and adapters for less than $400.
Money well spent in my opinion as my system sound excellent.
Good luck and let us know what u decide.