> In this case JACK's primary draw over PA/alsalib/whatever is solely the lower latency, for responsiveness in live performances.

I see you included ALSA as a library which you claim wouldn't hit as low a latency as using JACK. Is that what you meant to write? If so, what situation do you have in mind where sound arrives at your ear faster by way of app->JACK->$foo->speaker vs app->ALSA->speaker?

I ask because I'm only familiar with using JACK where $foo = ALSA backend. It stands to reason that JACK-handing-off-to-ALSA cannot possibly achieve a lower round-trip latency than going straight to ALSA. And the round-trip latency measurements I've read to compare those two routes confirm that reasoning.