know there's a huge impedance mismatch (the mixer is roughly 120 Ohms out, while the 520s is - will a matching xfmr be enough? I'd like to hear from someone who's done something similar and who has had to deal with the level and impedance mismatch.
Maybe it doesn't really matter.
It matters, for quality audio.
Professional audio technicians deal with Impedance (Z) matching as well as
Balanced and UN-Balanced Inputs / Outputs on a regular basis.In RF, you deal with Antennas.
At antenna feed-point you use BAL-UN or UN-UN and Z transformation (1:1, 4:1, 9:1).
Coaxial cable is an UN-Balanced transmission line, BUT a Dipole Antenna is a Balanced radiator.
Whether you are working with RF or Audio -- they BOTH follow AC Theory and are both Electromagnetic Waves.These principles have not change since discovered in 19th century.
Bob Heil explains the basics in Heil's Tipshttp://www.heilsound.com/amateur/tips/
Balanced to Unbalanced Wiringhttp://www.heilsound.com/amateur/tips/balanced_to_unbalanced.php
All Things KenwoodAmateur radio transceivers and transmitters use UN-Balanced audio inputs.
The Kenwood Hybrids (TS-520S) desire a High-Z (impedance) input.http://www.heilsound.com/amateur/dspsettings/all_kenwood.php
Audio Matching (Z in ohms) Transformers
Heil XT-1 High to Low impedance matching transformer, $17 USD.