That's impossible to do without a second sound card with stock opamps, in a second computer.
Humans have short memory when it comes to remembering exactly how something sounds. That is crucial when doing a comparison. Admittedly, it is a lot easier if the difference is clearly apparent.
When differences are subtle, the time it takes to remove the sound card, change the opamps, refit it, boot up and listen. Enough time has passed, that the memory cannot recall exactly the nuances and character of the sound, that was heard a fair few minutes ago.
It's very easy for someone to convince themselves, they can hear a difference.