Thanks for the very detailed response.
I did some digging around and at some point it looks like HP offered the Z420 with the Creative Recon3D PCIe card. I think there were some additional CTO options also.
I was thinking of initially going with a card to fit the legacy PCI slot (additional bonus is that it makes connecting the front panel audio cable to the card a lot easier than when using the top PCIe 1x slot) but these seem to be now rather old, with drivers / support pretty much abandoned by their respective manufacturers.
Currently looking at some of the lowe cards from the Asus Xonar line. The Creative SB Z looks pretty good also, but reviews seem to mixed.