Oscilloscope / dummy load strange readings

[qwerty1234]

Hi everyone!

I'm trying to get to the bottom of some inconsistent readings I'm getting while testing the output of an AC30. I have a signal generator (not isolated) hooked up to the amp, and monitoring the output through a 16R dummy load attaching the scope ground clip seems to load down the output. i.e. for a given input level (40mVrms) thru normal channel at max volume with just probe tip attached I'll read ~22Vrms. But when I attach the ground clip the output drops fairly significantly (to anywhere between ~12 to ~18Vrms depending on the probe and also seems to vary with me using different 16R dummy loads?!). I can hear the transformer audibly drop in output so I know the amp itself is reacting to the ground clip being connected to it. I'm suspecting a grounding issue somewhere but can't find it yet. It's no different if I disconnect the ground from the sig gen input cable (and just use signal).

Any thoughts welcome, thanks y'all.