First off,
Take a ****ing bow bro.
This is how you ask a question. Nice!
That is distortion, the fluttering.
- Are you feeding that via PC?
- Have you tried with a line out of a DAP to the iDSD?
- Drop all freq to floor across the board except for a peak at 55Hz as high as you can until the roll-off hits 0dB at the beginning of the EQ graph (20Hz?)
- Now max that.
You can compensate for lower slope with more gain since the other freq are flattened.
Are you still getting the fluttering?
FYI
90% of SZ issues had a PC in the chain. specs be damned it is a real observation so I always ask