Atkinson (Stereophile Editor) addressed this directly on www.audioasylum.com .
He said that the D rating applies to using the analog outputs and doing no upgrading at all, such as different power supply.
Personally, I think D is too low for the analog outputs. I'd go C at least and probably B. I find the tone is overall quite good, and what is missing is the last iota of detail that provides imaging and the kind of great nuance you would expect out of an A rated system.
Also I think he was a bit harsh in failing to note how he would evaluate it as an interface to a high end DAC. From what I read about experiences using it to interface a DAC, it should rate B at least and perhaps even A for that application.
He said that the D rating applies to using the analog outputs and doing no upgrading at all, such as different power supply.
Personally, I think D is too low for the analog outputs. I'd go C at least and probably B. I find the tone is overall quite good, and what is missing is the last iota of detail that provides imaging and the kind of great nuance you would expect out of an A rated system.
Also I think he was a bit harsh in failing to note how he would evaluate it as an interface to a high end DAC. From what I read about experiences using it to interface a DAC, it should rate B at least and perhaps even A for that application.