Keep in mind that the only thing the volume know on the Zx does is adjust the Windows volume so it's the same thing. But, if you want an external volume control and don't want to pay for the Zx, check out the Omni too. It's the same thing as the Z, just USB instead of PCI.
I haven't tried, but I assume it works the same as any other audio inputs in Windows. Click the little speaker in the lower left (system tray), then click "Mixer", It'll bring up all your volumes and you just adjust as needed. If that doesn't work, right click the speaker and go to "recording devices" insead of "playback devices". The Z software probably has sliders for this too.
Appreciate it Still,
I looked at the Omni and it looks like the ticket for control, with that said I think I'll still go the Z route just for the amping power. If I'm not mistaken the Omni is a little weaker. It could be a addition down the road to make controlling everything easier.