Mtothal,
The Creative Sound Blaster Recon3D is one I'd consider earlier, except that all the reviews said the same thing- sometimes fussy to setup and had to be reinstalled plus the sound quality was not terrific given a high price.
I also do not intend to ever use surround sound and combined with the cost of used (it's discontinued) Recon3D and various ASUS Xonar at least $70- $80+ in the US, it's difficult to justify. It's possible to buy a used Presonus Audiobox or Focusrite Scarlett USB for $60-120- and those provide high controllability, full recording, editing, and MIDI while preserving a precious PCIe slot. As the z620 has two GPU's- one double height covering the PCI slot, a PCIe M.2 drive (HP Turbo Z 256GB), there is only Slot 3 which is x4. Already, there isn't a spot for the LSI RAID controller of my dreams (LSI MR9260-8i), which are PCIe x8. The problem witht the z620 is the ungainly 2nd CPU riser compresses the area for the card slots:
I didn't plan properly as the original idea was to buy a Quadro M4000 (8GB) instead of the Quadro K2200 /Tesla M2090 combination. I haven't even finished the z620 upgrade and already I'm looking for it's replacement. It's silly to have a dual CPU system providing all those PCIe lanes and they can't be used well.
Digital sound at this level is not too demanding and PCI has plenty of bandwidth and works perfectly well. The consumer-level PCIe cards are usually only x1. Your concern of support - no new drivers for Windows 10- though is justified. But Wndows 10 does have legacy support and so far I haven't seen any dire proclamation that all PCI soundcards are simply unusable in Win 10.
What are using for playback? Do you ever record or edit videos?
Cheers,
BambiBoomZ