I have the SA8001, and will probably change-up to the SA8004 to run my computer audio through, while still keeping the one-box/ SACD function.
I only hope the 8004 is as good as the 8001. (I guess it is supposed to be better, but I note Johnjohn's comment.)
Running the same music from computer through DacMagic v. CD played through the SA8001 - no comparision - the Marantz DAC is great.