Hi, my first instinct when I read your OP was that a NUC that age was absolutely fine. Unless you have issues with usability, memory won’t improve playback performance. It is always nice to have but if memory is an issue it will be very evident!
I am not familiar with the latest tranche of hardware out of PS Audio, I have a couple of older pieces and the key to their performance is the i2S via HDMI input they pioneered. What you need is a DDC. USB in, various output formats supported. Companies like Singxer and Matrix Audio make them and the profile of PS Audio means their particular means of i2S over HDMI is well supported. The darling of the PS Audio community is the Matrix Audio X-SPDIF 2. It won’t break the bank but it will clean up the USB and convert it to a format that will maximise your Gain Cells performance. If you want to explore power options do it with this rather than the NUC. The MA should cost no more than £400 with the included switched mode PSU. They are being advertised for less on a couple of sites. On a slightly cheaper note, Holo Audio make their in line USB conditioner and that might improve your situation for around £100. That will be an import job though.