Amp that has continuously variable bias?

Excuse me if this has been covered before.
Is there a power amp that varies the bias according to what the power demands are? When the speakers are demanding more current , the amp will up the bias to remain in class A, and when the current demand is low, the amp will decrease the bias to save energy and reduce heat.
I know that many amps remain in class A up to a certain level, but that's not what I'm referring to.
Is this what the Krells do? Any others?
What you are talking about is called "sliding bias." Some of the Krells do this (the ones with names that end in an "S") although I don't think the new ones do. YBA also uses this scheme and a few others that I can't think of right now.

It is a good idea but hard to execute - that's why it isn't commonly used. I remember reading that some magazine reviewers weren't big fans of Krell's sliding bias amps, saying that large-scale dynamics were compromised. But I know the YBA amps sound fantastic so it is doable.

I thought it was called "sliding bias". A search turned up very little.