I've run line in a few times with the u89s/m148... the u89s output a very, very low signal.. combined with the m148(+20db) i did NOT have to flip the -20db switch.
Friends with schoeps have reported the same and Bean ran 481->m148->V3 for a Phish shows also without having to engage the switch.
I'd be more concerned if running line in from a SBD