I'm looking at these subs: http://www.onlinecarstereo.com/CarAu..._DBXi-15D.aspx

And looking into tuning them with these radiators:
http://www.sonicelectronix.com/item_...d-SLAPS15.html

Anyone here have experience with passive radiators?

 

If you have room for a 15" passive, why not just add a true 15" sub in its place?

 

The passive radiators require no extra airspace in the box and they allow you to change the tuning frequency at will. The earthquake subs sound good in a 3.5-4.2 cf box, so I really don't have the room for four of them. And besides that I'm sure that this setup will be loud enough.
The question is should I build a box with all of them facing up or two facing the back and two facing up?