Because the HD192 and the UAD1 are working totally different parts of the signal chain (the HD192 providing I/O from the outside world to your PC, and the UAD1 providing effects processing in support of your DAW software), there should be no *direct* compatability issues. They are skinning two completly different cats.
You do need to make sure you have enough open PCI slots in your PC for both of them, of course.
However, the $64M question with the PC platform always has something to do with software drivers. In this case the question would be are there any problems running both the UAD1 and the PCI424 software drivers simultaneously? I can't answer that directly, but - as much as I hate the state of technology when it comes to driver quality here in the Stone Age of computing and am always skeptical - my instincts would tell me that there is probably no issue between the two drivers.
However, my instinct often worth a hill of badly burned CD-Rs. Might I recommend talking to somebody who deals and supports both of these pieces. Maybe a local dealer who may have both pieces set up in their showroom, or who would be willing to try it for you since the commission on a UAD1 is probably pretty damn good
. Or a solid bet might be a phone call to one of the sales engineers at Sweetwater (even if you don't buy from them); because of their volume of sales and support with a large support database at their disposal they can often be an excellent resource for answers like this one.
HTH,
G.
