I use a couple of different Ebtech devices for impedance matching.  I use the Hum Eliminator if the levels are the same on both ends, and I use the Line Level Shifter if they are not.  These are passive devices that don't require batteries or (yet another) transformer plugged in somewhere.  They are solidly built and work well for me.  I got mine from Amazon.com.  I looked at a number of different products but the others seemed oriented towards folks who worked more in the analog domain.