It sounds like you loved the Sound blaster Z but didn't like the mic quality, did you disable the mic options in the panel or did you disable the mic boost which some times messes with the quality of your voice. I found with the omni that i had to disable The mic boost and other options before my voice came out clear, even when I had the omni on my desk away from my mouth, they was still able to hear me, but that was because I had the option on that allow you to talk softly and still be heard. Also some of the voice changer options also can also cause issues too. Did you try another PCI-E socket? I have a friend which uses the Z as well and uses a mic and he doesn't have your issues as he turns off all of the voice effects and mic boost.
As I found the DG/DGX to have muddy bass which was also boomy and you might not enjoy it since you loved the Z that much. I would suggest you try the suggestions I mention before shipping the Z back again. Going external might solve your issues as I have had similar issues but re-guards to noises in the audio which was gone when i switch to the omni. The major thing is the U7 has Dolby home theater V4 which is very poor software if your looking to use virtual headphone dsp. If you have no plans of ever using it, then that wont really matter. But a Used omni that you linked is $49ish dollar's, The only thing with the omni is that it only supports 48khz and 96khz playback, meaning you might have to use a re-sample plugin if you use foobar 2k, tho you might not have to if you use the default Direct sound option in foobar.
Edit: the sound blaster omni is the replacement for the X-FI HD Usb which was on the market. the Omni just has a better headphone amp over the X-Fi HD/pro Usb.