Thanks very much for taking time to make those measurements, I appreciate it.
This is the transformer:
https://www.sowter.co.uk/specs/9063.php
Sowter say no unbalanced current allowed, which is not too helpful but fair enough. Also they give total dc resistance referred to primary as 106 ohms, but no figure for the primary alone which I think is what we need to know?
With a bit of hunting I found this data for a different transformer, which gives a view data points for the effect of dc current:
https://www.sowter.co.uk/specs/9100.htm
This suggests that even 0.125 mA increases the LF distortion significantly.
The 9063s are on order, I won’t have them for a couple of months, so I can’t do any tests on them.
Were your measurements taken with no load across the outputs?
I wonder if I could use balancing resistors to null the dc offset?
By the way, thanks for pointing out that the digital signal can have dc offset, I had never considered this.....