What’s the relationship between gain (dB) and power (watts)?

My Question:
Setup is active speakers and pre-amp.
Speakers have input gain knob for -10 to +10 db.
Which is best, crank the volume on the pre amp upto 80% or add gain on the speakers and less on the pre-amp.

Btw. Pass XP-22 and Adam Tensor Delta  speakers

That Bryston gain switch would be to match the volume of two disparate amps on the same preamp.

MC running a clinic here. Nice. 

A master clinic involves the recognition that in power amps variable gain is just variable negative feedback. See Roger Modjeski for an EE w ears and many successful designs the variable NFB RM-9 being a sonic master class. The cool thing about variable NFB w time and phase accurate speakers is that the intrepid listener ( given a decent recording ) can discern how NFB effects image depth....
carry on....
see also Burning Amp

Often just variable NFB but sometimes just R....