Although i`ve never performed a real blind test, i`ve done test like this before and noticed the same things.
I made mp3`s, and burnt them in wav format, then played em in car.

128kbps gives u bass which is "different" I cant explain why exactly, it seems exagerrated and boomy compared to the orginal. (my mates get in and marvel at how much bassier it is if i have 128k mp3`s playin, i dont like it, it sounds fake)
The top end becomes swirrly and swishy, imaging is poor (excuse the techy terms :))

192kbps is good, bass is pretty accurate and top end swishyness goes, something is different about the sound, but unless listening back to back with a wav i couldnt noitce it. Imaging is nearly perfect

HQ Lame VBR (about 180-230kbps) awesome, high end is perfect, bass stays unchanged. Soundstage perhaps "lifts" slightly, imaging is spot on, imo this is what eveyone should use, its almost perfect.

256kbps, Huge files, perfect sound, same soundstage points as above.

I used either lame or Opticom Mp3 Producer 2.1 (ffraunhofer (sp?))

its the classic quality v filesize thing, i would always use 160k or more, and i`d never ever even if i had no other enocder, use xing :)
Xing gave me swisshy sounds and big bass at any bitrate, cant stand it.