Ive owned 2 sundowns for about 2 years now. If you set it up right, both will have good sound quality, and both will get loud. I can tell you from personal experience that if its done right, sundowns will sound a little bit better than Alpines when both are in the same price range.
if youre referring to the $400 type Xs, you will probably get better sound quality out of them then the sundown sa 12s. If its in your price range, I’d say upgrade and go with the sundown X12 subs.