Further to my above post, I had a Bryston 4B-ST for six years. I left it on 24/7 except for electrical storms and times when I would not be home for a week or more at a time. When I would power it up after such down times, it would take a day to reach full performance.
Assuming the amp is used in a revealing system (and that is an assumption here), I am not aware of any solid-state amp, not even an amp with a stand-by function coming out of stand-by (my Rowland 6's, for example), that will sound completely right until it's been running at full idle for two or three hours, and this is the exception. It can take a very long time for caps to fully charge and for the unit to fully come up to temperature.
If a person is not hearing the difference between a solid-state amp that's only been for a very short time and one that's been on for a few days, some limitation elsewhere in the system is the cause (or all that time he/she spent on the firing range shooting clay pidgeons before getting into audio ... :) ).
