I have tried sound cards with a replaceable op amp and you generally cant tell any difference. While I am sure you may see something if you run it through an oscilloscope, but Nothing that at least a pair of a $100-120 headphone can detect.
What sets a soundcard apart, is how well it can process the audio.
For example back in the day, when the audigy 2 ZS cards, The EAX console really improved audio quality as it not only allowed you to improve certain levels of the headphones, eg boosting the bass, and balancing out the audio to try to get a flatter response. It was able to prevent clipping and other issues which were common with onboard audio at the time, and even today.
eg the audio in my audigy 2 ZS is still better than the onboard audio on my gigabyte 990FXA-UD3 board. (especially with the modded drivers which brings back some of the features that the card had with windows xp.
(pretty good for a card that I have been using for nearly 10 years through all of my PC builds)
For the replaceable op amp asus has cards like that, and I tried them and was unable to tell any difference.