with about 10wpc and 5 1/4" speakers rated at just 88dB with their ports plugged, i wouldn't exactly say the system cranked, but it could compete with smaller boomboxes, especially in the sound quality department with sturdier wood cabinets and REAL tweeters that were pretty fast and detailed.
i managed to get a woman sun bashing up and dancing to "play that funky music" from about 100 feet away on a bike path, and, unlike car system owners, had a woman freaking on my leg to "my sharona" downtown once with that system.
it was just that little "15wpc" class D amp underneath the CD player running off 8 AA batteries. at full volume, before distortion, i could get about 5 hours of runtime and sometimes could feel my bike frame vibrate when playing, but it would get drowned out anywhere closer than 50 feet to a nightclub.
the amps, originally called sonic impact are now re-branded by dayton as DTA-1 and cost $30, but they were dirt cheap when they first came out, $15 i think. they were really raved about in hifi magazines when they were still called sonic impact for both how cheap they were and how good they sounded. i wouldn't suggest one though if you're using one as a portable because their cheesy speaker terminals break easily and the rest of the plastic parts are kind of flimsy too, but with a wall wart, they'd be awesome for a desktop system or driving the kinds of high efficiency speakers fans of 1-10wpc tube amps use. you can get reasonable volumes out of 90dB speakers, but you won't shake the walls. some people were swapping sonic impacts for expensive tube amps for the sound quality.
these days, if you want to build a cheap bike system, ou can get even cheaper amps like this Kinter MA-170
for $8.30 or even less. i like buying from parts express ,myself as i've always gotten good customer service from them. small class D amps are generally overrated for power. like the dayon, this one rated 18wpc is probably closer to 8-10wpc. with amps like the kinter and $30 lepais (used to be $20) though, you'd need to build an external power supply.
you can get even more powerful amps though if you just buy the modules and DIY. you should use lithium ion batteries with class D though for their 3v output as especially at higher power ratings, class D amps like 24-36 volts or even more. high voltage makes them happy.
class D amps are tiny and twice as efficient as regular class a/b amps at 80% or better efficiency. i'm surprised that they aren't talked about left and right here by people modding their boxes with class D for more power or efficiency.
i like the sound of class D too... it's generally very crisp and detailed with excellent bass control, but some people who like warmer sounding amps think they sound lean and sterile
the trailer system i want to build though will REALLY crank. i'm planning on using a pair of 2x8" wedges with sweet sounding dayton titanium tweeters for sattelites rated at 93dB on 40-100wpc class D modules and probably putting a 97dB 12" wedge under the trailer pointing down with a 100w amp, all running off just 8 D cell sized li-on batteries at 24 volts. that system should really crank and not get drowned out near clubs.
But wouldn't have it any other way.
2015-08-11 15.38.26.jpg
from 1980
That sounds like a scene straight outta Venice Beach 
Thats very cool right there, pop that baby in and adjust the vertical hold and enjoy!