passive radiator
 

I have a sealed box that used to house two 12's, but only have one 12 now. I'd like to turn it into a ported box, but space for the port is just not there especially with the box's position. I was thinking about a passive radiator, the CSS audio adjustable 12" seems like a perfect option, but I've never used a PR, so I'd like some feedback as to the SQ and output. Thanks.

Sq is as good as any other system if done properly. The biggest problem is tuning as it's a little tricky to add mass to the cone (that's how you adjust tuning freq.). Also, most passive radiators need to be accessed from the rear to add weight, so it can be a pita. However, that new css apr looks really good and has received great reviews so far for it's adjustability. Another issue is the over excursion of the pr, so you have to make sure you use one with enough xmax or sd(cone area) to offset this..the old rule of thumb was 1.5-2x the diameter of the main driver, but i think the css pr's have a lot more xmax so the 12" should be fine with that speaker at it's rms max. Another disadvantage is cost as you have to buy a pr which will be more expensive than a port in most cases.

If you do end up getting a passive radiator, You'll need to cut that internal box volume down since a PR takes up very little internal volume vs another sub, or a port, so you might end up having to rebuild a box or chop down the current one.

Yeah, depending on the volume of your box...that may be necessary as too much volume may cause over excursion of the pr and the primary driver as well. If you have to reduce it, you can always use foam blocks glued to the inside (they are light, easy to calculate volume, and easily removable if needed in the future).