Whammy headphone amp

Hi guys!

Build the amp and have an issue. It works great with the Audeze sine 18Ohm but distorts and low frequencies with the 35ohm focal elegía. I tested with 18R and 35R load and confirmed the 35R load produces distortions at the lower frequencies. So I cheated and placed a 18R resistor on each output on parallel and the focal sounded great as well.
I have done a version with irf610/9610
And a Fairchild version with the FQP’s
Both have same issue.
For op amp I used the opa1611

Schematics is same as original, only went lower on negative feedback to 3K because it had too much gain

Any help appreciated