I want the truth about 12s

From my experience, I've had a 10" Pioneer in a ported box,a 12" Clarion in a factory built Clarion enclosure and a 15" Kicker in a sealed box.

The best one,IMO,was the 12" Clarion. That thing sounded amazing. I listed to lots of rock and rap with it and it hit SO hard and the sound quality was amazing. It had a Clarion amp with it,but I don't know how many amps it was,since I bought the sub/box/amp used from a guy.
I had a Panasonic HU and 4 Kenwood 4-way 6x9s that were 280w. That system was just killer. I could have it cranked up,with the windows up,doors and trunk closed,walk across the street and still hear the song perfectly.

I'm personally looking into using 4" components in the dash of my Mazda,maybe 3 or 4 way if I can get them that small,3 or 4 way 6x9s and either 1 or 2 of the square Kicker subs.

oh yeah,the 15 I had was in a small sealed box and I didn't like it,so I got a bigger sealed box and still didn't like it. I had a 1000w Power Acoustik amp powering it,and it just sounded like ass compared to my Clarion. I fiddled endlessly with all my settings to try and get it to sound as good as my Clarion to no avail.