Ok, so I have an update sooner than I thought I would - I ended up RMA / returning the card 
The problem was that with the card plugged in, the Z420 was making a high pitched whinning noise. More likely than not it was some sort of electrical interference - maybe from the wireless mouse or the nearby router. I didn't notice this at first due to testing the card out a higher than usual volumes and sound quality was overall excellent, however once I did it became really annoying and I decided I will not put up with this, especially since on the integrated Realtek audio there was no such problems.
I would have two further comments with regard to the card:
- the Asus drivers aren't the best. E.g. with Skype installed, when shuting down the machine the program would crash and return an error message. There are alternative freeware 'unified' drivers available but frankly the manufacturer should ensure that everything is functioning well on the offical drivers
- if you are running Quadro SLI with 2x two-slot cards than you would want a PCIe sound card to put in the top PCIe x1 slot rather than the bottom legacy PCI card since this will not allow you to put a two slot card in the bottom PCIe 16x slot. This is pretty obvious but also easy to overlook in the grand scheme of things.
I will probably look at one of the Sound Blaster Recon3D PCIe cards now, which were in the HP quick specs. t least there is more hope for compatibility, although he problem is that most of these cards are not available as new anymore and there is always the question of driver support under Windows 10 (seems a lot of manufactures are still struggling with this).