So I received my Powercab Plus Saturday and wanted to share a few things I've seen.
1) The cabinet seems very bass heavy, possibly something Line 6 can fix with a future firmware update, right out of the box I turned on the global bass cut on the back of the cabinet, everything sounded much better at that point, it cuts the bass by 80hz or something to that effect.
2) Output of the Flat preset seems to be fairly quiet compared to loaded IR's, go into the settings whether on the interface of the plus or via the powercab edit app and increase the level of the Flat preset to it's maximum +6.0db, again maybe this is something Line 6 can address via a firmware update as this setting seems comparatively quiet compared to the loaded IR's which all have their levels cut by something like 14.0db and when increased this thing get's pretty loud.
3) Run the cab at full output and adjust the volume via your modeler, also you may have to adjust the output of your patches to get a desirable volume level, I've been able to run it this way without getting a clipping warning from the input light on the cabinet.
I haven't tried my 3 Sigma or Celestion IR's but I would imagine I'll find similar results in level as the pre loaded IR's, this thing seems to be aimed at IR use or like I said maybe it just needs a few firmware updates before it see's it's full potential.
I hope this helps and if you have any suggestions please respond as I'm trying to figure it out as well :-)