So, after using the first version for a few months I've decided to rework the design and put in the features I think I'd like.
The first change is to add a Linkwitz Transform Circuit to allow a bit more bass extension. The goal is to get it down to about 50 Hz or so and I tested the drivers last night with a 55 Hz signal and they survived pretty well. The way the circuit works is a little complicated but the upshot is it artificially boosts the low frequency signal to overcome the driver's natural roll-off. Essentially what this will do is give me some serious bass coming out of a tiny 2" driver. There's a ton of information about this out there and you'll find copies of an Excel spreadsheet that will help you calculate the values you need. But I found an even better spreadsheet that will simplify your design by giving you the choice of using exact values, with components in series and parallel, or using a single "close enough" value component and what sort of deviation from your goal you might expect. http://audio.claub.net/software/LTwithMC/LTwithMC.html
(I had already spent nearly a week trying to design the PCB layout for multiple values, and selecting the value of components to put on it, when I came across this. It would have saved me sooo much time.)
In order to calculate the LTC I had to know precisely what the dimensions of the box are going to be. And since I was optimizing I may as well make it as small as I could. The main constraints here are the battery packs and the size of the amplifier board, as well as the width of the BlueTooth decoder. The final dimensions came out to 10.5" x 3.25" x 3", 102 cu in. (The old version was 12.25 x 4.25 x 3.25, 169 cu in. which is a 40% reduction in volume.) It'll be mocked up in 1/4" MDF and cut on the laser cutter. It'll also get a much better paint job and perhaps a grill on the front to protect the drivers and the power switches.
Another upgrade is using a BlueTooth/FM/MP3 USB/SD card decoder, w/ Aux and a remote. I've only ever found them on Ebay, and the price is ridiculous cheap for what it does, $10. It should be arriving today or tomorrow. http://www.ebay.com/itm/400596390388?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
The bonus feature is the USB port can charge your device, but I'm sure it's in the 500mA range, if it's even that high.
I'm considering adding a few super bright LEDs to the front but that might be taking things too far.
Power supply upgrades will be a dc-dc boost/buck converter that can take any voltage from 3-35V in and output the exact voltage I need to charge the batteries. The old charge circuit required at least 14.3 volts input in order to get the 11.6v I needed to charge the batteries, with a 2V drop across the LM317 and a 0.7V drop across the diode. I might be able to really play around with the solar panel voltages with the input voltage this low. The other power supply upgrade is a USB buck converter that can handle up to a 3A output for fast charging. I might be able to add some resistors to put out the correct voltage on the data pins to allow Apple products to use this full amount of output. Another hope is that this will help with the ground loop issues I was having with the initial design. I'll be trying to find some small switches so I can turn off the USB port and the amp and BT circuits.