Hi Radcam
I never seriously followed up getting a usb>spdif converter - life got in the way.
As it happened I got the chance to listen to a friends 555PS on my NDAC in my main system. It wasn’t optimally set up but it wasn’t clearly better than my La Scala DAC and in some aspects arguably worse.
I can get my La Scala upgraded to mkiii level, the NDAC is as good as it is ever going to be. If I’m going to max out a legacy item I own I decided it makes more sense to do it with the Naim CDX2 and I’ve decided to sell the NDAC after hanging on to it for several years. I still think it is a very good performer (and if it had a USB input it would be staying) but I don’t see a use case for it and someone with an ND5XS2 should get to enjoy it.
I’ve bought a Rega DAC-R (which I picked up for under £400). Not the most transparent device in the world but very musical and it really seems to suit the Nait 50 - together with the EX its a very nice three half-width box system for a reasonable outlay.