I swapped out my sub amp a few weeks back- 2006 Alpine V12 for my 2004 Alpine V12. Completely different animals, and a mixed blessing at that. I wouldn't have thought they'd be so different from each other. While I can't say the new amp sounds bad, it does have less punch than I've become accustomed to (in spite of being 600 watts vs 400 in the old one).
It gave me an excuse to recycle some equipment though. I bought a Momo MM2124 DVC for a friend (powered by my old V12 monoblock) a few months back, to go with a new Alpine CDA 9855 I'd picked up in December. Everything went in today, and there will be a new 4 channel and replacements for the stock Monsoon component speakers in the future.
After installing everything, I finally got the chance to actually hear the Momo sub (because most dealers near me never seem to have a display model to demo). Even without a chance to break it in properly, I can say it sounds pretty damn good in a sealed box- better than I would have guessed, especially right out of the packaging. Very nice balance of sound quality and output. Sadly, the woofer will be a 17 hour drive away by the time it's properly broken in. I should have bought two :). I used to prefer 8 and 10 inch subs over anything larger, but now I feel pretty confident that an SR 124 DVC will be in my future.
As for the Alpine 9855, it isn't as intuitive (to me, at least) to dial in as my older model HU, and the manual definitely doesn't seem to be as good. The crossover and EQ options seem to be better, and the sound quality is excellent. There is one new one left at a local retailer, priced for top dollar, so I'm tempted after playing with one most of today.