Turns out this may be another case of user error. User error seems to show up in many of my experiments but I haven't nailed down the root cause of it yet.
I took the Line 6 completely out of the mix -> same issue.
Guitar -> Valverb -> Amp...this gave me rich, luscious reverb.
So I re-read Vic's post and started tinkering.
In the original setup I had the Trio's Preamp Out jack going to the speaker amp with the Valverb in the Trio's effects loop. When I switched so that it was the Mixed Output jack on the Trio going to the speaker amp (Valverb still in Trio effects loop)....voila. Problem solved.
So....my guess is that the mixed output jack leverages the effects loop, but the pre-amp out on the Trio doesn't. Is this correct?
If that's the case, then why would tinkering with the Valverb levels have caused sound issues... wouldn't the whole effects loop have been bypassed?
Vic...you know the Trio well. Any thoughts.