I've been on a few SoCal bike party rides (C.R.A.N.K. MOBs). Super fun!
I've also spent some time thinking about what it would take to make a good music bike. Your selections tell me that we think alike: it is all about sound for the pound. Sensitive speakers and efficient amps. Neodymium woofers.
I'm not a speaker designer, but I know just enough to have some (perhaps stupid
Those full range drivers have neo magnets and are pretty sensitive for dynamic drivers. And not having to include a crossover will save both weight (inductors can be pretty heavy) and complexity. That seems like a pretty good choice, IF they can play loud enough and have wide-enough dispersion.
However, I do wonder if you might get better sound, more sensitivity, and more SPL from a CD (compression driver) and a neo pro midwoofer. You could use an active crossover. Perhaps even a really simple one made with a handful of electronics parts (I think), since the speaker does not need to be super accurate. You could run just one of these 2-ways pointed out the back of the bike. Or maybe point one out the back and another out the front. You could run it mono. Use one amp channel for the CD(s) and another for the midwoofer(s). Use a Sonotube for the midwoofer enclosure. You'll probably want a neo CD, too -- CDs with ferrite magnets are heavy little suckers. This setup might be kind of nice in that you could blast sound out the back and/or front without it being deafeningly loud for you, and you might even be able to hear someone on the bike next to you.
Or maybe there is some better way to do this since you probably don't need as much output as a normal CD? Here's a not-so-huge speaker design with high sensitivity, lots of SPL capability, and wide dispersion:http://www.billfitzmaurice.com/Wedgehorn.html
Perhaps it could be useful to think of this as a 3-way, with the "sub" playing up higher than most home theater subs play -- something like 200Hz, maybe higher. And then the challenge of making a high-output, high-sensitivity "top" might be a lot easier. It does seem like the pursuit of bass extension is what causes a "top" to be relatively large.
If you go with a ported box for the sub, a Sonotube is surely the best option for the enclosure. They are pretty light and they are strong because of the shape. A box with the same volume would almost surely weigh a lot more. You could have two and mount them like bass canons on the sides of the bike.
However, at the expense of more enclosure weight, you could get more sensitivity (= less battery, and perhaps higher SPL) from a horn-loaded sub. Here's a small-ish one, but it is still pretty big:http://www.billfitzmaurice.com/autotuba.html
In the interest of maximum sound for the pound, I wonder if it is possible to design a horn-loaded sub that uses a Sonotube as the outer surface? Maybe there is some way to install some divider boards (and bracing) within the Sonotube to produce a bass horn. That seems like it could be an interesting design challenge.
Alternately, making a horn that divides so that it can be placed on a rack over the rear wheel of a bike could be interesting. It might look like a combined trunk bag + panniers. Or maybe the horn extends only down and out from one side (flaring as is descends), and the other side can hold batteries and electronics or something. That way it wouldn't have to "split", which seems like it would be hard to do well.
MORE TERRIBLE IDEAS:
This isn't really a performance-optimized idea, but I think it would be interesting to make a fiberglass speaker enclosure that is part of the bike frame. You could make a weird shape - extending up, down, and/or out the front, back, or sides to achieve the required volume. Paint it up to look cool. That low profile sub driver you specified could even fit in the front triangle without being too wide between your knees. You could do something like this for the "top", too, instead of or in addition to the sub.
If you are carrying some big ass batteries, I don't think it would be cheating to add a hub motor.
You could also add some fancy lasers, spoke lights (it could be like a video screen!), EL-wire, etc. that actually respond to the music. Maybe even a cheesy smoke machine? Disco ball?
Maybe a thing where people can txt songs (a link to a youtube video?) and the bike will download and play it. Or maybe they can text you messages and the bike does text-to-speech on the messages between songs. Or maybe have a display screen (scrolling LED, or the spoke lights) and the texted messags would appear there, rather than messing with the sound.
Have two bikes: L and R, and have each one play a single stereo channel. One is master and the other one gets its signal wirelessly. Ride in formation to produce stereo image for those in the right place(s). Bass from one or both bikes.