After my little fiasco jumpering the input caps, I decided to do an all-out upgrade.
NOW the amp sounds fantastic! Pre-modification, the headphone out jack on my $200 cheapo Sony receiver sounded BETTER than the Creek. Adding a regulated power supply only seemed to improve the SNR. The Sony receiver still sounded better. I then tried various opamps - LM4562, OPA2132, and finally dual AD843's. The AD843's sounded the best of the 3. I then tried a pair of OPA627BP's (normally $27/each) that I got cheap off ebay, but they turned out to be COUNTERFEIT - ironically, they sounded better than the AD843's!
My last step was to replace the input caps with AudioCap polypropylenes and all of the electrolytics with Elna Silmic II's, except the big power supply cap, which I replaced with a Nichicon FG. I also replaced the counterfeit OPA627BP's with real OPA627AP's from digikey. The only problem was that the caps were too big to fit in neatly, so they're bent into funky directions.
The headphone amplifier sounds way better now. Before the Creek's bass was weak, and the upper highs were muted compared to my cheap receiver. After the upgrade, the receiver now sounds considerably worse - "dry" and less pleasant. Bass is improved, and vocals sound buttery smooth. There is much more detail and separation in the upper highs. I'm very happy with the way it sounds now with my HD650's.
Total Cost of mods:
Elpac WM080-1950-760 regulated PS - $19
OPA627AP x 2 = $38
Brown Dog adapter = $10
Elna Silmic II and Nichicon caps = $20
I hope this additional information will help someone. The OBH-11's were selling new for <$100 on ebay a while ago, so this might be a worthwhile project.