Bear in mind the factors that affect amplifier "sound quality".....power, distortion, gain, frequency response and noise.
You haven't complained about noise from the Kicker that I've seen, so you gain nothing there by switching to the Rainbow. The Kicker has more power bridged (how I would use the amp) than either the 2-channel Rainbow in stereo or the 4-channel bridged....so you are actually taking a setup backwards by switching to the Rainbow. Both are capable of operating with inaudible levels of distortion, so you gain nothing here either. Frequency response I haven't seen measurements for either, but assuming both are competently designed there shouldn't be major deviations present in either so that should be a non-issue as well.
So what are you really gaining by switching or why would you expect the Rainbow to sound better ? The Kicker's are solid amplifiers, so there's no reliability issues to really consider.
If I had my choice between the Kicker KX650.4 bridged to my front stage and the Rainbow amps.....I'd stick with the Kicker.