So I have been thinking about this a bit more... The sax is far from a cheap pre-amp. They could have included a regulator to increase the input voltage to 24v. However, they chose not to. The question is, why?
Was it because doing it cleanly is difficult, and they wanted to give serious users the option of providing really good 24 v power? If so, is the performance compromised by the regulators included in the various batteries? I mean, there is a difference in designing a power supply optimized for a high end pre-amp vs. a variable output pre-amp built into a battery.
If I were a sax user, I'd be looking into providing it really good 24 volt power.
I have previously observed the impact of power quality on pre-amp performance by comparing the noise floor of the pre-amp via a detailed FFT spectrogram. Baudline, under linux, is an excellent tool for that.