Amp On or Off?

Bravo to Elizabeth for clarity.
There is compromise available. I have a Rowland which goes into standby when the 'Mute" button is pressed. In this position it stays on but only uses as much juice as a 100 watt light bulb. Surely other manufacturers offer this feature. Shop around.
The amp stays warm and the bill stays small. Everybody wins.

This has been covered many times in the archives. There are some threads that contain various explanations / points of view as to why some of us feel the way that we do. I would suggest taking a look at those answers, digesting that info and then doing some simple testing for yourself. For your convenience, here's one covering Class A amps, but most of the info can be carried over to high bias AB amps also. Sean
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comments & first hand experience with leaving amps turned on

Always leave it ON so that it remians stable.

It depends of the design of the amplifier. A good example is:
YBA amps are designed to be leave on all the time but not Gamut amps as the designer told me.

I use a Tripp-Lite surge protector. This, I hope, reduces in-rush current damage when amp is first turned on. For class A/B avoiding in-rush current seems to be the main reason to leave on all the time.