I have a DacMagic Plus with which I was initially very disappointed. It's performance was much worse than my Benchmark DAC1 -- completely failing to come alive and with a very poor sound stage. It was harsh with a poor perceived frequency balance. I was using asynchronous USB from a battery powered notebook computer running Foobar 2000 with the Cambridge Audo driver. The DAC is connected via balanced outputs directly to my power amp. I thought something must be seriously wrong.
It took me quite awhile to figure out what was holding it back. As an experiment, I tried removing the ferrite filter attached to the power cable coming from the switching power supply. Wah-Lah! The sound came to life, smoothed out, much sweeter and better frequency balance, plus the sound stage opened up. My power line is 120V 60Hz. To somewhat compensate for the removal of the ferrite filter, I twisted the power lead to make it into a twisted pair to help reject some EMI/RFI. My conjecture is that the ferrite filter interferes with the correct operation of the switching power supply. I imagine the ferrite filter was added-on after product design to help pass EMI/RFI interference tests. When I wrote Cambridge Audio about the power supply problem, a representative responded: "We have not experienced any significant issues with the DacMagic PSU but the bulk of our testing is done on 220v circuits so it is possible that there are issues to be reviewed on 110v."
With the ferrite filter gone, I am hearing some music magic at times, but the system performance has been very inconsistent, with tremendous dependence on the quality of AC power in my house, and what's coming from the power company -- early morning usually providing best sound quality. Plus, for best sound I have to unplug the DVR, microwave, portable phone system, wireless computer network router, and desktop computer UPS. And, this is with the DacMagic Plus PSU being plugged into a PS Audio AC line filter, and having a separate noise filter plugged in parallel with the PSU, and all this is fed from it's own dedicated AC line with its own house circuit breaker. The power amp also has it's own separate dedicated line and circuit breaker. I may need to upgrade the power line coming to my house.
My DacMagic Plus PSU might be defective, but I'm suspicious that other owners have been experiencing similar problems. Some of the descriptions of performance for those who have rejected and returned a DacMagic Plus, or have found it deficient, sound similar to my experience. I think Cambridge Audio has some troubleshooting to do.
If you have a DacMagic Plus, I strongly suggest you try removing the ferrite filter. It's very easy to remove once you figure out how to unlock the clasp that holds it together. There are multiple loops of the cable inside the ferrite filter that must be unwound. Acquiring or building an alternative 12V DC PSU, maybe even a battery supplied PSU, might be a revelation for this unit.
I hope this helps,