focal, jl audio, mosconi gladden, rainbow, hertz, arc audio, to name a few. the kappa series is fine for an upgrade, but to be honest, there are a lot of die hard infinity guys that swear by them. however they are many others out that sound better. they are good as a cheaper alternative and have no complaints about them, however we do not carry them.
I actually think the regular infinitys are not good at all but the kappas are something else. Im not desigining a show car but with the right amp and crossover you will get clean crisp sound that can get loud and give great mid to high range. Thats all I am looking for, don't recommend them for subs though......I'm all in on the jl's for subs. I used to have 2 12" w6 v2s....with the right box and amp thats some massively deep sound.