I installed a system a dozen years ago, and have fine-tuned it ever since. Presently has a pre-amp with mixer to mix sounds from 3 devices, with a separate volume control so I'm not stuck using whatever the player device provides - I-pods suck on the volume control - I usually use my Zumo 550 for input to it. Has a 400watt Blaupunkt amp under the seat, and 4 Alpine speakers in pods on the handlebars and mounted on the sissybar staunchions. Makes all the volume I want to hear, at all speeds. And I left some of my hearing in the nam.
I left the steps of the install, including the original parts and the improvements - if you take the time to read it you can avoid some mistakes. Matching the speakers to the amp matters. So do refinements like bass blockers (large capacitors), power filter, and a ground loop isolator.
http://www.horseapple.com/Valkyrie/Tech_Tips/Jammin_with_Blaupunkt/jammin_with_blaupunkt.htmlYou can see 3 of the 4 speakers in this shot:
