here is what i think..its correct that you do need a x-over to work different sized subs... it's good that there is still guys who still understands how car stereo works....well to let you know in my opinion is to just run the 10's if not then just the 12.... its too much trouble to tune multiple subs....but it can be done...the 10's with a good box size and feed it with good enough power can really hit and trust me they can thump hard if done rite....
there is a real good site that i go to.... its
www.12volt.com... they should have enough info to help you biuld a box for your subs...
