Alon Wolf selected FirstWatt amps and a Pass preamp to demonstrate HIS flagship speaker which happens to be huge. I don't think scale will be an issue for the Magico Ultimate3. I don't think the FirstWatt amps or your Pass XS 150 amps would likely ever be compared in the same system by customers, but I'm sure NP has an opinion about their relative sonic capabilities. I remember a quote from him where he says something to the effect that more watts don't necessarily lead to better quality.
Magico used the X350 amp with the Mini speaker and the FirstWatt amps with his flagship U3. I think both are considered solidly in the high end, though they represent quite different price points.
The Magico Ultimate are ultra efficient speakers using an active crossover - IMHO we should not not use them for our usual approach to amplifier/speaker matching. And Magico used many other amplifiers - I do not remember of any endorsement of any amplifier brand as the official Magico amplifier. Most of the time public exhibition choices are arranged by distributors or dealers, not by the manufacturer.