It's a common complaint with the EH Holy Grail, among other things. Your Trio's gain stages are amplifiying the EH's digital reverb signal exponentially. You would get the same thing with any amp with multiple gain stages. That's the layman's description. If it bothers you, try putting it in front of the preamp, as in an effects loop. I found the Holy Grail to be painfully noisy. EH pedals in general have become very amateurish in both quality of build and perceivable tone. I've checked out your rig and your playing and touch are exceptional, very good stuff. I'm surprised you're using the Holy Grail.
Have you seen or used this before?
A cat had one at the NY Amp Show last year and it literally was the most amazing thing I'd heard in a while as far as outboard units go.