I agree with Bunkeromantik. I've only heard Sim Audio equipment at dealers, but I do own the X350.5. In my experience, the Simaudio products sound more neutral than Pass.
The Pass has the best of both worlds (SS & tubes). It has the midrange warmth of tube equipment, but the bass control of SS amp. I think one attribute that people don't talk about much regarding the Pass X-series is its ability to reproduce bass in a tightly controlled fashion. Don't get me wrong, the bass is plentiful and goes down deep.
I had switched from a Levinson 333 to Pass X350.5 two years ago. They were driving my then speakers Thiel 3.6s. Me and my friends were just amazed on all the improvements we heard. My friend ended up running out and bought one to replace his ML 433.
FrankC