Here's my two cents for what it's worth. IMHO probably not much to be gained from going from your platform to a similar albeit more recent one. You could expect to get improvements with a server mobo and low TDP CPU, but as you pointed out, the new Supermicro mobos with Intel Xeon D are pricey especially if you need more than one PCIe slot. What some of us on the forum have done is to get a Supermicro X11SSL micro ATX mobo (available for about 250 euros) and an Intel Xeon E3 1240L (or similar) from Ebay or Aliexpress for about 50 euros. In addition, the X11SSL has 3 PCIe slots, DDR4 and integrated graphics (the E3 1240L doesn't), providing a cheaper solution. Mobos like these have good quality VRM, which is sometimes the case on gamer mobos, but less so on others. Stable voltages help good SQ. Another advantage of using low TDP platforms is obviously easier cooling in fanless cases but more importantly less 'work' for the power supply and potentially less noise generated by the mobo's components.
If you want to get a genuinely huge improvement in SQ with your current set up, then install JCAT DC cables for your SSDs, CPU and JCAT cards. That would be money well spent and you could be surprised.
I hope these thoughts can help. Your internet site is pretty cool. Kind regards.