Help with Mini Sub filters?

This should do what you need, it's the only dedicated active crossover design that I know of, though there are probably others around the price is not bad and it comes as a kit if you prefer that. Other than that there's always passive but I assume the Boominator Micro already has it's own amp, batteries etc.

Is there a particular reason to choose an A/B amp for the sub? They're not very efficient and for sub especially that'll drain the battery very fast. The operating voltage (up to 25v) suggests it's going to need a 24v battery to produce 25 watts - 12v would give much less, maybe 8-10 watts?

This amp seems worth a look for this sort of application: http://www.dx.com/p/y148-audio-amplifier-module-93121 it can produce 20 Watts with 12v, or 30 if you can get 15v into it, and it has a built in clip limiter. (needs a heat sink added for 4ohm)

Edit: 

For a much more hands on option if you're up for a challenge - sourcing individual components and so on, there are more options, building a board from scratch. I'm researching similar myself at the moment but it's pretty daunting stuff!

Given that you're going to need in total: An amplifier, a speaker, a crossover, and a power supply, it may be that the best way to cut costs is to build a new sub rather than try to squeeze a decent output out of your old one.

That said there may well be a DC powered amp and an active crossover hidden inside the old sub so it's definitely worth a tinker if you know what you're doing.