Maybe just the DAC section uses the LME49720. I see no mention of that opamp on the non-DAC model page.
Changing the opamp is easy, you just take out the top 4 screws (you'll need an allen wrench for the front ones) and then pull the top off. Then you pull the opamp out and replace it with another. Make sure the opamp goes in the right direction. The opamps have a little indent and a dot in the top left corner, and I believe those should be facing the amps inputs (the back of the M-stage). So if your looking at the front of the M-stage with the volume knob facing you, the opamp should have the indent farthest away from you and closer to the rear of the M-stage.
Like this:
One last thing...
I get the sense that your already pre-biased towards thinking that the OPA2134 opamp is bad, but IMO it sounds perfectly fine. I think some people take any issues they have with the sound on a piece of gear and then blame it all on the
first interchangeable part. I see this happen a lot with the Fidelio X1 - people listen to it and then blame any issues they have with the sound on its stock cable (with it's
1 ohm of extra resistance
). All their issues with the sound seem to magically get fixed as soon as they use a different cable, and the cable becomes the scapegoat for everyone's issues (the other day a new X1 owner said he heard a
"night and day difference" when he switched out cables). After this process of people blaming their problems on the cable repeats for a while, people eventually start labeling the stock cable as "lousy","garbage", "trash", etc. New people who get the headphone and read others negative remarks often just conform to the idea that the cable actually is garbage, and they voice that and the cycle repeats.
I think the same thing happens with the M-stage and its stock OPA2134 opamp.
Feel free to play around with changing them, but don't be suprised if the differences in opamps turn out to be very minor.