Could be something the amp is sending a lot of gain then ive got a master volume control. Although that other time i tried no fx loop.
Are you saying it might function better with a buffer? I used to use an active volume pedal (ernie ball MVP) that i can still try and dont care less about buffered bypass in fx loop. Coincidentally it splits into a good sounding tuner out too, so maybe even as my active splitter (would only control one channel) if it doesnt.