Just to follow up, I ended up ordering a Asus Xonar DG PCI card.
The key considerations were:
- relatively inexpensive
- overall highly recommended / good reviews
- opportunity to utilise the legacy PCI slot, in close proximity to the front panel HD audio cable
- headphone amp circuit
- optical out
- dedicated Win 10 x64 drivers
Once I recieve the card I will post some thoughts on how it compares to the standard onboard audio.
Just to confirm, my understanding is that before I install the card I should turn off the onboard Realtek audio? From what I recall I would do this in the bios, Security Settings then 'Azalia Audio' or something similar - is that correct?
