Unless your prepared to do some trial and error...
your biggest hurdle is going to be quality. It's difficult to buy or build a passive x-over to match the characteristics of the driver unless some engineering spec's are known. It's a shot in the dark! You can try a prefab passive from an autosound dealer, or Parts Express has some that go down to 80Hz or so but again, you may not get optimum performance out of the sub. Another thought would be to invest in a plate or remote amp that has a built-in adjustable x-over but for $100, you will still have a quality issue. The amp doesn't have to be mounted on the sub, just a box about a foot square and 4"deep will do. Personally, I would do a dedicated amp for the sub as your mains and the sub will probably sound much better at elevated sound levels. With a dedicated amp, you can use a plug-in module inline between the Adcom pre-out and the sub amp in for about $25-$30 if the amp doesn't have an onboard x-over.
If I think of anything else, I'll let you know.