was the thread on here? i did a bried thread search over by Line 6 and I did find some posts reporting clicks and pops with X3. however, Line 6 does not explicitly report incompatibility issues with Max or PC platforms which would lead me to believe that the issues people with OSX Leopard have are mainly dependent on settings which need to be tweaked for X3 and OSX to work well. Unfortunately, I do not use Line 6 products to record direct into my PC or my Mac since I use a dedicated audio interface for both so I am not much help there.
I would suggest scouring the boards more for some further insight into this concern because I seriously think it is more of an individual phenomenon (personal settings) rather than something which concerns OSX explicitly. To avoid the "he-said-she-said" problem, call Line 6 to have your questions answered directly. Hope everything works out.