90% of my listening with the MKII is with my iPhone 4. I've run it through the UHA-6 MKII, Objective 2 (all paired with the ODAC and/or iPhone) as well as my desktop DAC/amp (TubeDAC-11). I've found the improvement to be very marginal.. even when I've focused very closely on it. Better control and tighter bass seems to be the common theme of improvement through all the rigs.. but it was, arguably, 10-15% better than what I heard straight from my iPhone. Dynamic range and soundstage showed some improvement but, again, not to any notably markable degree.
Of all the IEMs I've heard/owned, the GR07 MKII seems to improve the least when used with additional amplification. I do agree that better DACs & source file quality has a distinguishable impact on sound quality, though.
One thing that bears mentioning again.. I'm truly impressed by just how natural the MKII sounds.. I've used the GR07 MKII for quite some time now and can confidently say, while setting technicalities aside, I like it more than the W4.