What I purchased my NDS I had an XPS-DR. The dealer informed me that the non DR 555PS would not necessarily be an outright winner over the XPS-DR. He said that some things would be better with the 555ps but not everything and the XPS DR would win on other things. I think its a muscle over refinement difference from what he said. The advice was not to ‘upgrade’ to a non DR version but stick with the XPS DR.
Of course the DR version is a different matter and there was no question in the dealers mind as to the superiority of the 555PS DR in every way.
I suspect the above would ring true with the NDX2 as well, perhaps even more so as the NDX2 was designed and released post DR PSU’s whereas the NDS was designed before the DR technology.