Tech specs:
Windows 10 (21H1)
Sample Tank CS 4.1.4 (21E19)
Apollo Twin USB running UAD software v9145
I often use software like SampleTank in stand alone mode, just for quick relaxing jams before bed. No need to fire up the DAW for that.

Where the strange things happen

Before I get into the weirdness, the following DOES work:
- Running SampleTank within ProTools, I can set the output of the track I have SampleTank on to "Virtual 3/4" and it works great.
- Using the stand alone versions of similar sofware, like Kontakt, Waves standalone instruments and GigPerformer, works fine so it doesn't seem to be a problem with the Apollo.
- Running SampleTank in stand alone mode and using the Apollo's standard MON L and MON R works just fine.
The strangeness when running SampleTank in stand alone mode:
- Choosing Virtual 3 and Virtual 4 as my Left and Right channel outputs results in complete silence (I already use Virtual 1 and Virtual 2 for my ProTools output).
- By accident, while taking the screenshot above, I discovered that choosing Virtual 3 as Left channel and MON R as Right channel results in a wet reverb on the Left, with a dry Right side. So, that kinda works.
- However, choosing MON L for Left channel and Virtual 4 for the Right, only results in a dry Left side, with no output to Virtual 4. It does NOT do the expected "flip" that you think it would.
- Just for kicks I tried Virtual 1 and Virtual 2 and they behave in a similar manner. There is a pattern here - no output with both Virtual 1 and 2 together, a "wet/dry" output with Virtual 1 and MON R selected, and only a dry Left side when running MON L and and Virtual 2 together.
In summary, I can't get Sample Tank to output to any stereo pair of "virtual" channels. Only odd virtual channels (1 and 3) seem to work, and only on the Left side! Yes, I know I could simply insert reverb with SampleTank itself, however, I really like using the AKG one. It's quite lush.

Additional notes:
The Apollo Console software lets you "link" your Virtual outputs into stereo pairs for ease of use, if so desired. I tried them both linked and unlinked, restarting SampleTank after each change. It made no difference.
I also tried other instruments beside Art Deco Piano. No difference.
I appreciate any help!