I did some more tests between the UK F886 and UK ZX1, and the findings I made earlier are consistent, repeatable and don't need any kind of blind listening test.....its ridiculously obvious.
1) The ZX1 has more bass presence. I am not sure if this is because the amplifier is better or simply that it is bass boosted! But you don't need to convince yourself of the difference, it is plainly obvious.
2) When volumes are set to max on both units, the volume is quite similar. However, at 2/3 volume on both units, there is an obvious increase in volume on the ZX1. It is as if they have non-linear volume scales. This would suggest the UK ZX1 does still have a volume cap but perhaps only a slightly more generous one combined with a different volume scale
3) When you connect the AKG 3003i to the headphone socket of the ZX1 with no music playing, you hear a short hiss for about 1.5 seconds. When connecting the same IEMs to the F886, no burst of hiss is there
Because of point 3, it would suggest the electronics are different between the units. And point 1 could be a forced EQ on the ZX1 or once again due to a different amplifier circuit. Or it could be as OK-Guy suggested, that the crippling EU volume cap is affecting the whole sound, and not just limiting the volume.