One thing to pay attention to while blind testing is that people are amazingly sensitive to level mismatch. A very small difference in volume is often perceived as a quality difference in favor of the slightly louder source, so if you want to do an accurate test, you'll need to make sure that the two are as close as possible to perfectly volume matched (unfortunately, matching by ear isn't really good enough to be sure that the difference is inaudible - you would need to use a meter). If you can't do that, it could still definitely be worth testing, but the results aren't as guaranteed. One possible way around that would be to start at zero volume in every case, and turn it up as desired from there (during every test). If nothing else, that removes any consistent bias to one or the other. Of course, you'd have to be able to turn each one up without realizing which was which...
I looked up the impedance of your Beyerdynamics, and they look relatively flat (with a small jump in impedance, but not in the bass). So, their frequency response is unlikely to be hugely affected by output impedance. Depending on the design though, there could still be a change in the bass - if they rely on electrical damping, they will be underdamped when used with a high output impedance, which would cause the bass to become more prominent but lower quality. So, that's still a possibility for what you are hearing between the two.
