Mark, I once had a Sony ( traded it to Max ) that had 4v preamp outs, it would drive the amp just fine at good volume levels and no distortion.
I now have an Alpine installed that has 2v preamp output, I tried it and couldn't get any volume. I hooked up the speaker wires to the amp and now it works fine.
So, I guess it all depends on how good the preamp p-p voltage is.