More power for moderate listening levels?

I'm hoping the higher power amp (15W Class A before switching) shares some of the sonic character of the current amp.

Class AB amps don't 'switch' per se. The slide into Class B. Depending on the design and the load, the transition can be smooth and less so.

Getting AB bias currents exactly right as the amp plays is an engineering challenge. It almost always lags the amp's thermal state. Manual bias amps are frequently mis-biased from the factory as the amp is set up with the covers off in an environmentally controlled lab. Close it up and stick in a rack and all bets are off.

see https://www.zenmod.in.rs/pass-labs-x150-5-checkingadjusting-offsets-iq-and-gain/ for a Pass 150.5 bias adjustment tutorial.

Like many other recent 'inventions', 15W Class A before switching is nothing more than ≈30mV bias across 10x 2Ω emitter resistors in ± amp halves.

Pass is a brilliant designer with an impeccable pedigree, so his designs are likely SoA.

@ieales, it would not be polite to tell you what I think of Tekton loudspeakers. 
I am pretty sure most speakers are destroyed not by raw power but by clipping. Certainly you could just plain overpower a driver but most people do not have that kind of power. 
I am not an electronics engineer but my JC 1's biased for 25 watts class A get hot enough that you can not keep a hand on the heat sink. That has got to be a lot more than a few mV bias. 
Who in their right mind would put a class A amp in a rack? Even millercarbon would not do that:)

The "transition" for my lack of a better term for class A to B operation is really interesting. Certainly, the class A watts do not "go away" but get "added to" as more power is needed. Curious how the output device can operate in both modes. Am I thinking about this right?

That has got to be a lot more than a few mV bias.

The JC1 has 9 pairs of output transistors.
25W ≈ 22mV across 18x 1.3Ω emitter resistors