Okay, another opinion, for what it’s worth…
Adding a psu to XS3…
Running a HC2 with our XS2 and find that to be very nice 
At some future point, might be tempted to use our XS2 with a newer HCDR, (if or when the pre-loved prices drop to a certain level).
Just wasn’t convinced or even tempted by the FlatCap, when we did the audition.
Details previously posted here…
Adding psu to nDAC…
Lots of anecdotal posts, on this forum - over years - suggesting a similarly small gain, but an unimpressive step forward with XP5 on nDAC. Maybe you’ve already found these same comments ?
Having done own research and several listening tests too, own mind is already made up for at least XPS DR or maybe 555PS, in time. Personal experience is this gain is worth the funds required.
Good luck with this aspect of your journey.
It’s all good. We all make different decisions, based on a bunch of both objective and personal factors.
Interesting to see which direction you choose. So, following with interest.
Good luck.
R