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

A few say that Pass amps are better than preamps, not that preamps are no good. I heard the same about Rowland, Lamm and others. It appears that it is more difficult to make a great preamp than power amp. Who is going to make great preamps ? Let me guess - just a handful of tube guys, correct ?

127 degrees is not the normal operating temperature of my amp ( X-350.8 ). Just checked it with a heat gun after 4 hrs of play. It's 88 degrees.

88*F=31C which is probably too cool.  Nelson has posted elsewhere for older X series amps that 45-55C on the heat sinks is reasonable (along with what that subjectively feels like by hand in case you doubt the thermometer), and the manual for your 350.8 specs a temp of 53C (127F), so Pass recommendations seem historically consistent. Your amp could probably stand to have the bias increased a tad. At least on my old X150, the trimmer pots are so fricken sensitive that lifting the amp, carrying it across the room and gently setting it down will mess up the bias and offsets, so I readjust after I've moved the amp to where I want it. I shoot for 48C in winter time, so given the ambient temp changes in my man cave it will run about 50C in summer.