I sure don't want to argue with @ellajeanelle, as obviously is a nice person, but I am involved with pro and consumer both and I cannot figure out what you are talking about with "voicing" of speaker outs vs line level (-10 RCA) or pro level (+4 balanced). ALL sub set ups in professional applications are +4 XLR balanced, this is the single safest way to do it, straight from the source preamp, using a "Y" cord to split the preamp output between 2 destinations: #1 to amp (then to main speakers) and #2 to the sub.
Maybe by your phrase "carrying voicing" you mean some of these highly colored preamps, like an old tube unit, where the line level out is already changed based on the tube circuitry inside the preamp? That would be a case where the line level output DOES carry the color of the preamp as the only circuitry inside the preamp is tube. But in the case of most modern or solid state units, the line level output labeled "tape out" or "preamp level out" or "subwoofer out" would be relatively color free.
If you use the speaker level sub input, aren't you adding the preamp output color to the amp color to the speaker cable color and finally to the sub?
Brad