What are the weak points of Pass amps and pre-amps ?

In my opinion, talking about amplifiers without mentioning the rest of the system, e.g., preamp, speakers, etc., is pointless. Most of the “engaging “ aspects of the sound quality comes from the amplifier pairing with the speakers., and in the right room. I’ve heard several moderately priced “systems” that were a lot more  engaging than systems costing multiple times more. 

I had a 350.8 so I'll address the heat issue. I found it surprisingly warm to the touch. But it did not heat the room.
I would have loved to have kept it but it was just too large for my small apartment.
As it is so small I am very sensitive to heat and stuffiness issues.
The temperature of the Pass in no way affected the atmosphere in my room. Others may have a different experience but this was mine.
Had it not taken up so much real estate(you really don't know do you unless you have it in your personal space)I would not have parted with it.

and the First Watt, X, and XA series all sound different...

jetter, I am almost certain that Pass would not be good enough for me, definitely not when high - for me - cost is involved. I would have to wait longer and go higher. Pass seems to be a solid choice, that's all.
As for the Ypsilon experience, there was something terribly wrong either in that system or with the listeners or both. Some do say that Ypsilon may sound a little bright and forward, but it depends on the rest of the components and preferred sound. Stage III cables are often used with Ypsilon.

Hi inna, I completely understand regarding cost.  I am thinking of what I believe is the least expensive amp, the XA25.