I couldn't tell the difference on my Creative inspire 5700s. The dolby decoder in them packed up so I changed to my Sony stereo amp, with both Sony and B&W speakers. Suddenly all my 192kb/s mp3s sound somewhat lacking.
Have also been reminded how pap the amp on my speedlink medusas is. Aside from picking up RF and EM interferance, music through it sounds really harsh. Plugged the same headphones into the Sony amp, stuck the headphones in Stereo mode and they suddenly sound half decent for music! Clear as day too, with no interference.
I've not experimented with bitrates yesterday but with a decent soundsystem, it suddently seems worth looking at higher bitrates or lossless.