Oooo...
Crossovers are a long and complicated subject. You'll find some good info digging thru threads on this forum, but you'll have to dig.
The main point for an open baffle crossover is this: You have to fight the rising mids of the woofer. Or falling bass, if you look at it that way. So most woofers on a reasonable size baffle will have this rising midrange problem. If you use a conventional box crossover, it won't be taking that rise into account.
To get a proper tonal balance you have to cross the woofer much lower than you would in a box. Why? Because the low crossover point fights the rising midrange and flattens out the woofer's response. The woofer will now have a much flatter response. In effect, you are attenuating the midrange to get it to match the level of the bass. If you don't, it just won't sound right.
What's the trade off? You lose sensitivity. You may have to trade away as much as 10dB of midrange to get close to flat with your open baffle woofer. And these low crossover points mean big, expensive inductors. How low you have to force the crossover point depends on the woofer and on the baffle.
Here's an example. Let's say that you have a 15" woofer and 6" fullrange on open baffle. You measure your fullrange and find that it's down 3dB at 160Hz. OK, good place to cross it over to the woofer, right? Yes. In a box you would just set your crossover point at 160Hz on the woofer and be done with it. But not on open baffle. You may have to set the woofer crossover point an octave or more lower to flatten it's response and get it to blend at 160Hz with the 6". So you may be setting the crossover point lower than 80Hz to get this to happen. Again, that's woofer and baffle dependent. But now that you've knocked down the woofer response, the 6" fullrange is too loud to match the woofer. You have to attenuate the 6" to match.
That's the short end of it - the very basic concept. It's a lot to chew on, but it should give you an idea of how crossovers need to be different on open baffle. There is a lot more too it, but that's for another day.
In the case of your crossover it is probably crossing the woofer too high for the open baffle. It was designed for a car, so it has it's own issues to deal with. You may not be able to reasonably modify it for OB. You'd have to choose you baffle size and measure to find out.
