it really depends on the send/return module - most are intended for guitar effects pedals - which expect guitar volumes - which are much lower than line level...
why do you think you need a line level output? a lot of mixers/audio interfaces can easily handle modular levels - possibly with just a bit of attenuation...
as for line input - all you need is a amplifier - some vcas can do this, veils for example - but most can't
maybe you could post a link to the rack?
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities