Let me start by saying that I was a bit baffled when I compared these two. The big difference I noticed was the weight, the Alpine's are built like a tank. I think the Alpines weight twice as much as the C12's, did not weight them but it felt that way. The image below shows the sweep of the two using the same everything, the only difference are the subs themselves. The sweep was done for 0 to 200Hz with the crossover set from open to 71Hz. The amp used is a PPI Art 600.2 bridged and no EQ. The enclosure is 0.9^3ft each, separate chambers. I ran two of each for the sweep.
I have always said the these Alpines are awesome subs by any measure, but since I am running the IA C6 components I figured I would give the Carbon subs a shot. I tuned them and gave them a listen. To my surprise there isn't much different in sound between the two, both of them drop nicely and blend very well with the components. Both give more output than I will ever use. I listened to Daft Punk Happy and I had to immediately toned them down, that track is very bass heavy. Once I lowered the level to my liking they sounded very nice. The bass was deep but not overwhelming. On Al DiMeola's Kiss My Axe there is a point where the bass hits hard, no problem. These subs play them without any issues. This is the type of song where it can make a sub crumble when played loud.
There are a couple of small but important differences between the two. I think the Alpine can drop a little lower than the C12 (graphs show otherwise). The other difference I picked up is that the kicks seem punchier with the C12 and the C12 seems to be a little more upfront. But then again, these were minimal differences, just enough for me to pick them up, or to make me think that I pick them up.
Here is the sweep, I leave up to you to guess which is which for now. I will reveal which is which later, I will not leave you hanging.
In all honesty, the C12's are much moreexpensive than the Alpines and even though they may have a slight advantage in SQ I don't think the difference in price is worth it (I am referring to street price). The bottom line is both are superb subs. I may have a pair of C12's for sale soon, or maybe a pair of SWR-T12's.