There are no hard and fast rules.
My 50wpc (class D) M-CR603 was
reviewed by Andrew with PMC GB1s (87dB, 8ohms, recommended amp power up to 200 watts and he found it...
"...proved more than capable of both driving them and making them sound rather special..."
And that was despite the M-CR603 being only very slightly more powerful than the minimum (40wpc) recommended by PMC themselves for those particular speakers.
I have heard PMC GB1is and FB1is used on the end of a NaimUniti and sounding superb. (With Jason Gould of Naim - on one occasion - taking great delight in giving it more 'welly' than I would ever dare to!)
There are also a lot of fans of class A amps (valve and solid state) who get great results from speakers with efficiency as low as 83dB despite their amps 'only' being rated at 25 wpc or so.