There are at least two links on his website that are articles/reviews of his speakers being used with 8-watt 300B amps .
I have successfully run my 88db Carbon 7 SE mkII’s with a 3.3 watt 2a3 SET amp and never was it close to running out of gas.
Remember, it’s that lack of a traditional power hungry crossover that allows the Fritz speakers to sing on just a few watts. The sensitivity isn’t everything. His speakers are a very benign load wrt impedance and phase shifts.
Give John a call and let us know what he says