I was just going to say buy an amp...

You can get a set of bass blockers and put them in line with your speakers. This may be what you already have in your 'high pass, in /out' filter. This is about as good as it will get on a factory head. To Dalton's point, a cap and coil (12 db) would be better, but its still a screen door on a submarine.

I'm actually a pretty big fan of passive crossovers, as there is lots you can do with them (DC protection, notch filtering, big slopes, impedance matching, Q adjustments, etc.), but they are somewhat of a lost art. Not to mention they do suck some power, negatively affect speker dampening, and can be a pain in the ass. Many (maybe even most) of the one's on the market today are not spectacular...they often don't use good components, they have arbitrary xover points, and often do a poor job of handling phase (there's more to it than just polarity).

Short story, go electronic wherever possible, use radio power, oh, never. If you have to go passive, I'd suggest building them yourself. At least get familiar with crossover theory, so you can maximize whatever the manufacturer gives you.

Toby