Given the difference in level between phono and line, the induced noise in the software preamp would in most cases be unacceptable for listening directly. It's provided as a last resort for vinyl time code users. So while it could be used in the external pass-through mode, it would likely sound like crap (unless you had a $1000 sound card or something with an extremely low noise floor,) so wouldn't be worth doing.
Mixxx Developer - Controller Specialist
Mixxx Windows package maintainer