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

I remember running a Yamaha 1970s CA 1000 integrated amp with 15 watts Class A switch.  It got rather warm but it did sound superior to the Class A/B 70 watts (about).  The 15 watts sounded more musically involving, smoother and quieter than Class A/B.  

I own an EAR 890 which is 70 watts Class A.  It gets burning hot on the chassis and transformers after 30 minutes. 

I now use a pair of monoblock 130 watt tube amps running Class A/B.  It is a voltage regulated design which runs so cool that after 2 hours, you can place your hand on the transformers which are merely warm.  Plus the sound is superior to the EAR 890.  

I don't think there is an advantage running Class A over Class A/B, depending on design.  The cost of powering Class A and the heat are definitely a consideration.

Note, one could fry an egg on a hot EAR 890 amp, it’s that hot for the chassis and transformers, dangerous to touch after 1/2 hour.  Pass amps currently list their temperature at 127 degrees, above warm and slightly hot.