For mic level signals, I use the Shure attenuator (Shure model # A15AS). It is a small tube with XLR connectors on each end, one male and the other female. It provides switchable attenuation of 15dB, 20dB, and 25dB. It, of course, is designed with input/output impedances for mics, that is, an output impedance of 150 ohms to interface with the input of a typical mic preamp. However, you could take one of these, disassemble it, and replace the resistors in it with others so that your input and output impedances would be better for line level use.
-Andres