At --alt-preset-standard, the files I produced were typically 4-5Mb in size whereas the MM files I produce are 5-8Mb in size...[snip]...bass and treble are usually the first things to disappear as you lower the bitrate and listening becomes intolerable below 128kbps.

Are you sure your version of Lame is up-to-date (namely 3.91 or 3.92)? I find that files made with lame --alt-preset standard are usually about 220Kbits/s average, and never below 180Kbits/s average. It's simply not possible to make files eight-fifths as big as that, as MP3 only goes up to 320Kbits/s...

Peter