Fast Track Ultra.

I used a Fast Track Pro, I think it's the same or a similar driver from Avid. I had to use it on location briefly, honestly I didn't really have time to properly troubleshoot. I had to do a fresh driver install since I had removed v 6.something from my system a while back. I downloaded the latest driver from the Avid site, and got a lot of breaking up sound with lower buffer settings unlike with the previous driver (I still kept it at 256 samples for safety usually). I ended up just kicking it up to 1024 or maybe even higher for the session and used the FTPs direct monitoring for the guy who was playing bass into it. Is that the kind of thing you mean?

There is a fix listed in the forum for this issue
 
The reason the Fast Track does not show up in your Audio Devices list is that the description in the REGISTRY is too long!
 
It is a VERY simple fix but as with any registry tweaking you need to be careful!
 
All you need to do is change part of a name from "Fast Track Ultra" to "FTU" (in my case) to shorten the overall description length
 
Here is all the info you need: http://forum.cakewalk.com/heads-up-for-fast-track-ultra-users-m2851213.aspx#2879368

From the X3b Update page, the first Audio Engine bullet point is:
Resolved a compatibility issue between the latest Avid Fast Track Ultra ASIO drivers and SONAR