the infity reference are built at entry level. They are good subs, they just dont have a high power handling or loud output.
The Kappa Series are nice. They dont get very loud, but they have decent power handling and a nice accurate, tight suspension. Great SQ with these subs. They just arent going to win you competitions.
The Infinity Perfects are nice as hell. Slightly over rated in power handling, but if you stick true to its RMS ratings they have a great crisp and clear response. And they can get pretty loud too.
Infinity subs arent the best thing out for everybody. . . but they are good products. Reliable, does what they claim they will do, and with a proper install anyone could be satisfied with them that is looking for a reasonably loud SQL system.