If it cost a bundle and you want it to last forever because there's a good chance it just might, leave it on. If you can afford to replace it when it dies and it is likely to fall apart anyway, turn it off and on.
On or off question...
For a solid state amp. ( Adcom gfa 555)is it best to keep it turned on ,or off . I listen about 10 to 15 hours of music per week. I also have an Audio Research pre amp ( SP 9 MK III) which I turn off because of the tubes . Both are protected .Thanks.