I picked up a Creek OHB-11 headphone amplifier a couple months ago. I used it at work with Senn 280's and it sounded pretty decent.
Then I saw how easy they are to mod. I used Nichicon audio capacitors for the output (470uF), feeback loop (47uF), and op amp supply pins (47uF). The main PS cap was upgraded to a Panasonic 3300uf. Input caps are Solen .56uF epoxied together and attached to the pcb using double sided tape.
An IC socket was installed for op amp rolling. The first one I tried was an OPA2134 and it was too bass heavy.
I used perf board to assemble my own DIY single to dual adapter that included a power supply cap for each chip and class a biasing using 2N5486/84 jfets. The opamps are OPA627. I had to stack two sockets for the DIY adapter to clear the feedback caps.
With minimal break in the amp sounds quite a bit better than stock. Especially the high end. Not sure if that's due to the 627's or class a biasing. Low end sounds taut and controlled.
I'm pretty happy with upgrades.