I received a PM from OPLx requesting info on the E-Tracker format. I have not posted enough to be able to reply to PMs, so here we go:

E-Tracker source modules are approx 80K and in a proprietary format - I wrote a brief description for Fred 34, see appendix B.
Most distributed files, for example as found on zxart.ee, have been compiled. This makes them significantly smaller and allows more efficient playback. A disassembly of the player code is available on my github.
These files can be decompiled, for this I wrote a decompiler which was published on Best of Fred. The source code is also available (see github).

Not so long ago Martin Bórik added support for importing E-Tracker modules (source or compiled) into his wonderful SAA1099tracker - see issue 17.