Is anybody using one of these mixers as a front end of the DAW with X1 successfully?

I use the Mackie ONYX 1620 Firewire with X1 connected to an HP Quad core. 12 gigs of RAM.
Never had a problem. I also use an RME Fire Face 800 with Fire wire without a problem. This leads me to believe you are having a 3rd party driver conflict.

That's the weirdest thread every, Asks a question 2 and a half years ago and then answers themselves? is there more to this thread that got deleted ?? 
 
Even weirder is the answer which seems to imply you solved this issue from 2011
 
http://forum.cakewalk.com/Exasperated-and-almost-done-m2402348.aspx
 
This forum is a great way to solve problems if you so choose to join in with us. 
Glad to hear your system is working now.