What limits MP3 at low bitrates?, any specific technical limitations? Options

[Cygnus X1]

With MP3 capability on about 6 devices that I own, I'm always looking to fit more than 10 minutes of music on a flashcard . However, the format doesn't seem to do very well below 128kbps. Why is this, and what technical aspects of the MP3 format limit it from achieving better quality at say, 80 or 96kbps? Can tools such as PNS or IS be used with the format at low bitrates?

Since Ogg and AAC support hasn't really materialized yet, and most devices support only MP3, it would be great if the format could be tweaked on the low end to sound a bit better. By this I mean "radio-quality" at 64-80kbps, 32Khz, 13-14Khz lowpass, IS. At these bitrates, LAME currently suffers from ringing, watery sounds, and flanging. FhG isn't much better, and if anything is worse since it uses 22.05Khz sampling rate below 96kbps. Even MPC's thumb profile sounds decent at 80kbps...which is interesting considering that subband encoders aren't "supposed" to do well at such bitrates. Why can't MP3 match this?

Any suggestions, or links to good papers on these limitations?

[Gabriel]

I'd suggest you to try --preset xxx on 3.93.1, with xxx as your target bitrate. Usually there should not be watery sound.

Right now, Lame is not very powerfull at low bitrates, mainly because of the lack of IS.