The following experience based on:
iMac 2011 >-- ifi iUSB power (Aqvox usb PSU) >-- Supra USB 2.0 cable >-- Meridian Director >-- Chord Cobra Vee3 >-- Earmax Pro >- Sennheiser HD800 (with DIY single ended cable).
Now I have 2 well known usb power supply in front. One is Aqvox power supply, borrowed from my friend. I have listened to Director with Aqvox for nearly 2 days so have a basic idea how the sound is with this combo.The other PSU is ifi iUSB power, which I received about 2 hours ago.
I would say, both of them, give significant improvements in every aspect. Really huge impact, very easy to distinguish. Darker, wider, smoother, better extension, more 3D stage, more detail. The improvement is just so big that you can't miss. If you want to me quantify the improvement, I would say, 25% at least. That is huge amount in hifi world. But that is just my point of view, maybe someone else has another opinion.
Between those 2 PSU, the difference is not that easy to tell. After 2 hour listening, I would say, the winner is ifi iusb power. For my system, it appears that, with ifi, the background is slightly darker, not too much, just a little bit. That is hard to distinguish. The dynamic seems to be a little better, but I am not sure if it is just my imagination. But one thing I am sure. With ifi, the sound is smoother, more natural, more musical. Apart from those, I can't tell any difference, in terms of detail, sound stage, extension.
Just to clarify myself, This conclusion is not bias because ifi is a little more expensive. You can find a cheap ifi iusb in asia for something like £115. And Aqvox PSU is 97 Euro. £35 difference is really nothing in hifi. And this conclusion is drawn from my personal hearing and my setup. It may vary with different person and setup.