Basically they're the same but there are noticeable differences. The Kappa's can accomodate more power and the newere models should produce better midrange according to its specifications.
The References are, for the price, extremely good at producing natural sound. Midrange is somewhat lacking compared to the Kappa's but unless you're extremely good at noticing it you won't even know it's lacking a bit.
I got the References based on it's price
erformance ratio. 605cs up front and 652i's in the back. You'll definitely need an amp to get the most out of these babies.