I have also had a problem with some tags from Foobar2000; a tag of “+0.61” is read by the Clip+ as “-49.39”.
I followed the instructions from this thread and removed the “+” sign:
http://forums.sandisk.com/sansa/board/message?board.id=sansafuse&message.id=23962
The Clip+ now reads the Replay Gain as “2070.61” and, of course, much too loud. I inserted a space where the “+” had been (" 0.61") and the Clip+ reads it correctly.
I installed Media Monkey and it does appear to write correct tags; I really like Foobar2000 and I don’t like having to install another program.
