I actually had a X350.8 with my X350.5 Amp together at the same time in my home. It was great to demo the X350.8 in which I was considering trading for and purchasing. The X350.8 was provided courtesy of Reno HIFI.
To me, and my system, the X350.5 was far superior in openness and soundstage. The .8 version was very dark sounding. Now this was when the .8 series just came out. I returned the .8 back to Reno. Soon after that, Pass stopped advertising the X350.8 in there line up. I think they also thought something was wrong.
It appears that they have since fixed what ever was wrong and now many report the X350.8 to be the better unit.