I've added a Kenwood power amp and four Polk Audio speakers to my bike, and thought I might share how it all turned out. If you click on the link below, you'll see the album with all the pics. Click on the first pic, and you'll see a detailed description of each pic on the right side, along with a scroll arrow to scroll to the next picture.
I completed this build with a cost of just $273.78 for the amp, and speakers. And believe me when I tell you - it sounds incredible for a motorcycle audio system. If you are a bit on the OCD side like me, then it will take you a couple days to wire it all up the way I did it - but you'll save thousands of dollars doing it all yourself.
I assure you - this setup sounds amazing. I'm a musician, producer and have made my living in the music industry for many years. My ears know what they hear and are very critical. For a motorcycle system and especially considering the cost - it sounds amazing. I tried taking a video of how it sounds - but it keeps distorting the audio on the video as soon as I turn it up. Believe me - it's freakin' LOUD. I don't even have the amp turned up halfway yet to break in the speakers.
I went with the Kenwood KAC-M3004 600W, 4 channel power amp. It's small, and all-digital. Got it on Amazon for $103. The Polk Audio speakers are DB-651 marine speakers and on Amazon for $65/pair.
Of course, there were some miscellaneous expenses for wires and connectors - but I already had all of that in my electronics tool box.
Here's the link for the pictures. Feel free to hit me up with any questions.