Can you run a quick test, turn on local monitoring?

[StuartR]

What would you expect the effect to be of running with local monitoring turned on.  Others in your session would sound like they're behind you, I'd presume, since you're getting the direct signal from instrument/mic into your headphones without even going through jamkazam or your computer.
I confirmed today that turning off the local "faders" on my device does not enable my personal mix sliders.
So leaning towards a Mackie ProFx 6 channel, or something very similar.

I think you've answered your own question. The whole objective of playing with others through the Internet is to keep the latency as close to being in the same space as practical.