[Alex at SVS]

Well, if your Pre-power combo doesn't have a sub out then I'd assume it doesn't have a way to handle the crossover from the Ushers to the SB12. From what you've described the only crossover available would in the SB12 (which is very high quality and won't add distortion). If you don't use that crossover then you're letting the SB12 and Ushers natural rolloff create the crossover, which won't be pretty and the transition between the two will be very rough unless that's a 1 in a billion lucky matches between speaker and sub where the rolloffs just happen to match at the exact same rate and same frequencies. Not likely though.