I have found with the N20 that you really need a good AES and USB cable if you are going to take advantage of DSD. I have read that the May has a great DSD section.
The N20 is a fantastic machine and I've found or rather heard no degradation in SQ by using the USB but saying that some PCM recordings sound better with AES.
The biggest difference however is getting an external 10Mhz master clock for the N20. This really does make a huge difference.