Yeah I think the XPSdr makes the NDX sound more like a high end analogue source (but not vinyl). I very much like what it does to the NDX. Some preferred it bare though. Depends on what you’re after.
As an alternative, you could also consider running a separate DAC off the NDX. Traditionally, people have done this with the NDAC or a Chord DAC, but there are plenty more to choose from. Not that there’s anything wrong with a PSU upgrade, but there are lots of options out there, and the only way you’ll find out which you prefer is to listen for yourself.
@Crifo couple of years ago now, I did half day with my dealer swapping out black boxes, for me the NDX with 555 was a real uplift, yes the XPS added but not enough to justify
Have not heard the XPSDR on NDX, I have always run mine bare, only ply NDX about 5% as source, but only true test as ever is demo,
I would try XPSDR vs 555 non DR, I have a feeling the 555 will still be the winner