kenwood ka 8100 runs hot when played loud on kef 104 .2

The old Kef 104.2’s (really great speaker when driven correctly) were a beast of a load for an amp, that needed big current from it. Sorry but your KA8100 just isn’t up to it.
To do them service those Kef’s would like (staying with integrateds) a eg (low cost) integrated like a used John Curl designed Parasound Halo Hint (early model), if you have the $$ then the Hint6 (later model)
https://parasound.com/hint.php
https://parasound.com/hint6.php

Cheers George

If this happened recently, or only one channel runs hot, then it's a problem with the amp. But if not, it's like George said, it's too low of an impedance on the Kenwood.