Hi Ross,
Here's a Facebook page that's quite active for Positive Grid users:
https://www.facebook.com/groups/PositiveGridGroup/

I'm not sure what the issue is with having no sound in JamUp when going to it from Bias - that's never happened to me but you can ask the Facebook group.

As for sharing presets... you can share an amp built in Bias while in Bias using Tone Cloud. You can also share a JamUp preset BUT NOT if it uses a custom amp from Bias. It's a real bummer and I hope they address that soon. I am assuming you are using JamUp XT Pro, and not the free version. The paid version allows easy upload and download of presets (with the exception being the custom amp thing).

I agree with your assessment of modeling power amp distortion... it's a pretty cool feature and quite effective, I think.