Wow, mikeisted, you're taking me back to the home-brew stereo days that most of us EEs went through! I think you're right, you're badly under-speakered right now.
A few thoughts and/or suggestions: First, keep in mind that a bare speaker cone in free air sounds MUCH worse than when it's mounted in a proper enclosure, so you may not be quite as under-speakered as you think right now. The bass response of an unenclosed speaker cone is essentially nonexistent, and the midrange is not much better. At long wavelengths (low frequencies) the air can just run around the speaker cone to equalize front and back sound pressure levels, killing the audio; obviously an enclosure prevents that from happening.
That being said, however, anybody who played in band in high school can tell you that sound just disappears outside, so you're going to need a LOT of sound to be even audible on the bike, especially if you want to be heard by people in cars around you. High power is very helpful, of course, and your 20W amp isn't bad at all. But it's also important to have a very efficient speaker, so when you're looking around, be sure to pass over any speakers with mediocre efficiency ratings. And remember that the most efficient speaker enclosures of all almost invariably include an exponential horn, which match the acoustical impedance of the speaker cone (high) with the impedance of the free air you're radiating into (very low), to minimize losses.
