Thanks for the suggestions. The problem was that I set the gains to achieve a target overal volume of 87db, instead of the default 89 db. All my MP3s, duplicated over three Empegs, a Carputer, and two desktops, were all at 87db. The software I know of for calculating replaygain and adding the corresponding vorbiscomment (flac encoder, or foobar2000) do not have a level option. They will just do it to 89db. I ended up processing the FLACS to 89db, and updating the MP3s from 87db to 89db with MP3Gain. Regarding my carputer and desktops it will be no sweat to transfer the updated MP3s, but when it comes to the Empegs I know its going to take a couple of days each. In the meantime, I'll leave the Empegs with the 87db MP3s until I get some more music.

P.S. The problem with having FLACS at 89db and MP3s at 87db has to do with mixing the two formats in the same player. I don't have space for all FLACS, so I have MP3s and FLACs mixed together (and playing shuffled).