I agree with mark_h. Windows users can try foobar2000's ABX component to avoid the audio memory problem in their experiment. I find that the crossfade feature of the ABX effectively causes more audio memory to be needed, so I disable the crossfade to better hear differences. By disengaging the distraction of audio memory, aspects of sound perceived by other parts of the brain become more apparent. The brain science is similar to how a person can shift their visual attention between central vision and peripheral vision. Focusing on the peripheral vision requires relaxing, which won't happen when the person is distracted by something in their central vision.
I find 320kbps MP3 to preserve the intellectual aspects of the music, but degrade the emotional aspects (which is why I listen to music). I find the difference even more apparent compared to hi-res recordings instead of red-book CD. Different parts of the brain involved in the two aspects. When I am listening for emotional differences between A and B, I find I need a relaxed training period before I notice them. Being stressed or distracted causes only the sympathetic nervous system to be active, while I need the parasympathetic nervous system to dominate to perceive the emotional differences. Once I am trained on these differences, I can focus on them quickly and do well on an ABX test.
Primarily where I hear the emotional degradation of 320kbps MP3 is in the bass. Make sure to listen with well-balanced loudspeakers or headphones flat down to 20Hz. This is inexpensive to achieve with in-canal headphones like the Etymotic Research HF5. Bass is better perceived with the full body (not just the ears), and this is least expensively achieved in car with a subwoofer. Getting loudspeakers to produce symphony levels flat down to 20Hz is hugely expensive in a living room.
Finally, having JRiver customers reporting on audibility of MP3 distortion is complicated by the variability in how muffled/balanced their playback system and recordings are. Trained experimental psychologists know to minimize such variables by comparing the results from difference subjects on the same playback system and the same well-recorded music. If MC is not configured as bit-perfect, then there is the muffling associated quantization distortion of digital volume control and/or digital filtering. Better to have the volume control in analog after the DAC so MC can be configured as bit-perfect.
Foobar2000 makes no claim to be bit-perfect, so it can be the dominant muffling factor in the experiment. MC, which can be configured as bit-perfect, doesn't offer instantaneous switching between A and B, so it can't avoid the audio memory effect today. I propose that MC is doing more harm than good by having a "listening test" feature with such an experimental psychology methodology flaw.