I haven't seen the actual diagrams of it all, but I would assume that going line in DOES bypass the mic preamp. However, if it adds 14dB of gain, it looks like the line input may be expecting a consumer level (-10dB) and automatically bumping it up to pro level (+4dB).
Of course it is entirely possible that the MT is just using the mic pre to do that as well, but most devices don't.