I've opened up my Behringer 2600 Gray Meanie and Black & Orange "Xmas Tree". Quick recap of the theoretical differences: The B&O has digital reverb, the GM has a spring reverb tank, and there were some rumors that the GM has "selected components" that made it a bit better in whatever respect.

Here are my findings:
- On the GM, the spring reverb tank is mounted to the back of the main board.
- On the B&O, there is a DSP that is responsible for the digital reverb. This DSP is not populated on the GM.
- Indeed, 6 of the capacitors in the filter have been changed from SMD to through-hole variants - so there is truth to the "selected components" story.