I bought this line insulator to protect my amplifier from the 48V phantom power from my mixing desk. I couldn't find any information from the amp manufacturer regarding the use of a transformer on the DI out of the amp, so better protect it than damage it.
I tried with and without (of course with the phantom power turned off), and I noticed no differences in sound quality. I also use two Vovox XLR cables.
Robust, good sound quality, does what you expect it to do. A tool which everybody should have, just in case...