But that still doesn't explain the constant change in volume. I  notice that when I turn the 'clear-bass' down / off in the EQ settings, this goes away, but then there isn't any bass - just treble and mids...

I got my Fiio E5 a few days ago, and I was using the 3.5mm --> 3.5mm cable that came with it while I waited for the L5 cable to arrive.   So this morning, I plug the LOD cable into from my walkman to the E5, and the first thing I notice is that the volume is suddenly deafening - I had the...