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.
i use passive subs w/an active outboard x-over - the marchand xm-9 deluxe - great product at a reasonable price. but, w/a passive sub, i don't see how yure gonna do this w/o another amp, unless there's some sorta passive low-pass filter ewe can run between the subwoofer speaker cables from the amp (run in bi-wire config.) and the subs, then running the monitors full-range, or perhaps w/a similar hi-pass filter on the monitor's speaker wires. but the only in-line filters i've seen like this are for interconnects, not speaker-wire, & ya still need two amps.
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.
The improvement with an electronic crossover and separate amp over a passive crossover (even with biamping) is considerable. Get one as soon as you can.