Mesa tube DI noisy

The DI on my Mesa d180 (forerunner of Bass 400) has a loud hum and louder hiss. The line out is no hum and a slight hiss noise. Can anyone tell me what might be wrong?

It's a transformer DI 600 ohms, taken off the speaker signal.

I tried the ground lift / filter , no change.
Tried swapping mic leads and input channels on mixer, no change.

I've had a search but not found anything relevant.