well, the ZX models are the new ones which replaced the KX line. both are great lines of amps. don't think either is better then the other, but the ZX line is not 1 ohm stable and the KX line is. the SX is a digital amp with a ton of features you may not need. it'll prob be more expencive then the other lines.
whats the VC configuration?? that'll prob determine whether you get the KX or ZX.
i just found out the new zx arent fan cooled, instead they use a new heatsink on the sides. this is a plus for me because i run two kx amps and their right behind my ear when i fold my 2000 2 dr cavalier back seat down with the amps attached, so when its warm at all and my volumes down, i hear two whaling fans. i cant wait to get my new zx amps.