are you saying the usb input sounds better than the spdif input?
That's usually not the case.
wonder if it's the similar on the DVA preamp?
I don't know that it's better. I'm running the optical output of a hifiberry digi pro into one optical input of the MK5 and my macbook pro into an OWC hub and from the hub's optical out to the other optical input of the MK5. I've got the coax digital output from a Sangean FM tuner going into one coax input on the MK5. I wanted to hook up another device that had only optical out so I decided to try the usb input on the MK5.
I may do an A/B at some point but I would imagine if there is any difference that I wouldn't be able to hear it. My hearing takes a nosedive at about 6-7k. It's always been bad, but it's getting worse with age - I'm 78. My hearing loss was the reason I waited until 22-23 years ago to buy my first piece of AVA gear. I just assumed it would be pointless given my hearing loss. That first AVA PAT-5 showed me most emphatically that it was NOT pointless, even with my damaged hearing.
I'll post my 'findings' when I do an A/B, but I don't know that I'd put much stock in them. Also - I've never had much luck attempting to give a critical listening to any of Frank's equipment - once the music starts I lose interest in trying to critique the equipment - I just enjoy the music. There are noticeable improvements each time I upgrade to new gear, but that's about the limit of my 'testing'.