Are you using polypropylene film/foil caps for your crossover? They aren't necessarily expensive, unless you buy certain brands. But the small values you need will come in polypropylene film/foil variants at very low cost.Check out the post called "the sound of capacitors." You'll probably be interested in that. I'm not fussy about brands - There are several good components from reputable companies, in my opinion. But I am a bit particular about the technology chosen. The link in the post illustrates why.