Leave class A on, or turn it off?

Hi,

I've long thought it is a benefit to both sound and reliability/durability to keep my electronics powered up all the time. But now I am auditioning a Class A piece (Sugden A21a), which runs very hot indeed. It's not obvious to me what's better for durabilty of parts and circuits: always hot, or frequent severe temperature fluctuations? So what's the way to go here? Leave on all the time, or power up for listening and down after?

I'd particularly like to hear from people who have left their A21a's powered up. It's that particular amp I'm concerned about, and while theory is nice, results speak lounder.

Thanks!