I'll be building my own music server in the next month. Having enough processing power for upsampling to DSD, and implementing fiber optic are priorities. All components will be in a fanless case to minimize audible and electrical noise.
The server will utilize a Solarflare fiber NIC, and a JCAT Femto ethernet card. I have a streaming DAC with RJ-45 input so will be running cat6 UTP straight from server to DAC. Fiber optic into server from either Uptone EtherRegen or basic FMC. Ethernet cards like the JCAT have upgraded clocks as well as LP4 Molex connectors to be powered by separate 5v power supply. No need for power from motherboard. JCAT, Pink Faun, Taiko all have USB cards with same features if USB output is needed.
RAM will be industrial grade ECC memory. Many disussions over the efficacy of ECC vs non-ECC. I believe most of the reported improvements come from the use of industrial grade memory, which has a wider operating temperature range and higher quality components. ECC less of a factor.
OS will be Linux based. Still deciding between Audiolinux, Euphony, or basic Ubuntu. Audiolinux and Euphony can boot from memory, so any noise from SSD is eliminated. Both also come loaded with Roon and HQPlayer embedded.
Power supply also still undecided. Fanless SMPS with DC-ATX converter on the cheap side, HDPlex ATX LPS more expensive, Taiko DC-ATX converter with unregulated LPS more expensive yet.
For me, building my own server is the way to go. Look inside most of these high-end bespoke servers and you will see motherboards from Asus or Supermicro, and processors from Intel or AMD. Their proprietary OS's are all Linux based. These servers are merely computers at the end of the day, most built with off the shelf components. DIY saves cost and allows for total customisation. You have freedom to switch components and software at will.
Of course this option not ideal for people with no ability or interest in DIY. For them a turn-key solution from any of the top brands is ideal. Nothing wrong with that. It all accomplishes the same thing.