Isolation of ANY component in your system means fewer ground-loops. Ground-loops are insidious things, not only creating hum, but also HF sibilance. Once they are eliminated, the noise floor drops considerably revealing much more of the music.
There are several opportunities for isolation, including:
1) USB interface - either with common-mode filter of isolated interface using the 5V on the cable
2) USB converter or transport - the S/PDIF output if transformer coupled eliminates another loop. The downside is that this usually doubles the jitter. It is actually better to have the transformer on the DAC S/PDIF input. This adds very little to the jitter.
3) DAC analog output to preamp or amps - if this is transformer coupled or isolated externally, this eliminates one of the most troublesome loops. Here is an external isolator/selector:
http://www.empiricalaudio.com/products/final-drive
Steve N.
Empirical Audio
