Most entry level car audio subs are 4 ohm, so check what ohm rating it is and just make sure to match your amp your using to power it with, and the subs ohm/wattage ratings, and youll be fine.
The only real difference between subs is enclosure type/size and output.
Since its already in an enclosure, Im sure its acoustically matched, unless its a prefab deal (ie: best buy) but as long as its close to the recomennded airspace youll be good.
Im not familiar with the BX5s, but if they have a sub out, that would run to the power amp that would be driving the sub.