The usual suspects... i7, lots of RAM 16 GB and more, GPU, SSD, etc.
Sound was good, just not equal to a high end CDP though. I always felt something was missing.
What did (and does) work is similar to what you've stated below.
Huge gains with...
1. NAS + Pi (running the likes of moOde and Rune).
2. Daphile on a dedicated PC.
3. 2 PC JPlay setup.
Next level gains...
Wireless streaming. Don't know if its because of isolation of electrical noise (that's my reasoning), but this finally gave me something I could live with and even say performs on par with my tape and CD setup and for digital music at 16/44 and 24/96. Haven't gone higher res than that.