I use a Marchand x9 too. I have two subs. The passive setups usually aren't real transparent. You have to pay close attention to the slope of the crossover. If the speaker is not fairly full range a first order crossover won't work real well. Contact the speaker manufacuters and if they are receptive they will tell you what you need. I contacted Martin Logan regarding one of mine and they told me the frequency and the ideal slope for this speaker the CLS2z.
x-cross overs HELP
I'm looking at a new sub and eventually a seperate amp. Till then I'll be sharing the fronts amp(BAT vk200) with this new sub. This is not a powered sub and I don't think I want to use the x-over from the sub company(their x-over is optional $50). Here's the kicker, I don't know anything about x-crossovers, let alone the best way to set them up. Any help would be greatly appreciated. Just cause I own audio equip doesn't mean I understand it.