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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.