I imagine the "differences" on the walkman are maybe down to the walkman and how it handles MP3 and WAV...the DAC in the 667 is better than the one in the walkman. I have a 667 too and it's incredibly good with music.
By the way. If you wish to surprise yourself even more... rip mp3 at 96, 128, 160,200, 256 and try again...
I would be very surprised if you can pass the ABX at anything higher than 128, and I can pretty much guarantee you won't at 200.
MP3 has had too much bad press, but if you use the right tools (EAC ec and LAME), then it's very high quality these days.
If you want a simple online test... here's one...
http://mp3ornot.com/index.php