I've been able to reduce noise and cut hum using Jensen transformer isolation devices. I've observed no adverse insertion effects from these. Here's a link:
http://www.jensentransformers.com/iso_aud.html
Check out their CI-1RR and CI-2RR products (about half way down the web page).
http://www.jensentransformers.com/iso_aud.html
Check out their CI-1RR and CI-2RR products (about half way down the web page).