Units like the Shure inline pads may not be the proper answer since they are designed to work in an "250 Ohms impedance world", while your line outputs are designed to work into a much higher impedance load.
It's not all that complicated to concoct a pad with a few resistors that presents a 10K (for example) load to the line level output while also presenting a 250 Ohm source impedance into the preamp. Here's a tutorial my friend Rick Chinn has posted:
http://www.uneeda-audio.com/pads/
The table at the very bottom of the page has resistor values to drop a line level signal to mic level.
Bri