[EDIT: Totally wrong description, I'll leave it to prove my incompetence

]
MP3Pro is an extended MP3 format; 64kbps MP3Pro has almost the same quality as 128kbps standard MP3, because MP3Pro stores the bass sections of the music files separately.
Standard MP3 codecs begin to remove bass sections when encoding in bitrates below 128kbps.
However, MP3Pro never became common, so I doubt there's a player which uses its advantages. You don't HAVE TO use a special player, but normal ones don't support the separate bass parts.