Notion 6 has received updates since I last spent time with it and I just spent the last couple of hours tinkering with it and got more impressed as I went. Initially I had been using it for its default instruments and, for someone who only RTFM when I hit brick walls, things like drums are not the most intuitive. I ended up revisiting the VST plugin manager, and found that you can now simply hunt down and add *.dll files to it and they add without issues. The help files say it automatically hunts down and adds VSTs from your default (registry) VST plugin path, but it doesn't (and that may actually be a good thing, since I want to navigate finding them without a massive list). Quick note: 64-bit Notion can only host 64-bit VST2 plugins.
I never thought to use Notion 6 as a host before, but in the drum frustration, I added Geist to the plugin manager and it loaded without issues. A nice side-effect for me is that I tend to work with onboard audio unless tracking, and Geist requires ASIO... unless it is inside a host.... and Notion 6 hosts it nicely and uses onboard audio nicely as well. Geist can then be used in pseudo-standalone mode ("Song mode" inside of Notion 6), so all work can be done there and Notion 6 simply triggers the whole shebang. Another upside is that everything can be saved within Geist, so Notion can be taken out of the loop later on.
After putting in a quick acoustic guitar riff, I ended up revisiting the plugin manager and added Heavier 7 Strings and TH3 to it, then went into the Score Setup and replaced the acoustic guitar with H7S and added TH3 as an effect (the best navigation to VST(i)s I found is from the Mixer view inside Notion 6). Overall was quick once I got exposed to that. Exported audio from the whole thing and went without a hitch.
Final test... tried to send to Studio One Pro, and got the popup that "can do this, but Studio One is not running"... so launched S1P, tried again, same loop. No data was sent from Notion 6 to Studio One Pro at all, even with both programs actively running in tandem. At that point I realized why I never got into Notion 6 before, but now that I realize Notion 6 can host on its own, I will need to revisit this issue.
Bottom line - Notion 6 has some nice hosting capability. The plugin manager, Score Setup, and Mixer View are very helpful for both work flow and program navigation.
[I will have to revisit why Notion 6 and Studio One Pro seem to be oblivious of each other, as that is a significant issue to me.]