I listened to your silence tracks on a Clip+. Both in Rockbox and the stock firmware they have the same faint background hiss on maximum volume, same for all three codecs. At normal listening levels it's basically inaudible (at least with the phones I used for testing).
I checked the FLAC track in Sound Forge, and it's SPL is apparently -71.2dB. So I assume it's not digital silence in the tracks?
EDIT: Made my own silence file, FLAC with -96dB digital silence. In the stock firmware it does hiss a bit at maximum volume (but less than yours), in Rockbox it's dead silent.