I have a Chord Qutest, and can vouch that it is quite good using the stock power supply; but also agree that the sq can be improved by using a better power supply.
For the last year I have been running my Qutest with an IFI Ipower Elite 5v power supply. This is the top of IFI's current lineup. It is an ultra quiet smps that competes with linear power supplies. It retails for $300. I also have a friend that was very happy using an Sbooster 5v LPS with his Qutest.