Well I’m certainly curious what you figure out because when gigging is a normal thing again, I was hoping to use my DI the same way.
I’m no professional on these topics but, did you try the pad button too? If you had the -40 engaged and/or the pad on the front engaged it could be a volume vs. noise issue?
In the SVT’s manual it says the tube direct output is not affected by the master but, maybe it is controlled by the gain? Just thinking it could be trying too hard to amplify too little? The manual also doesn’t tell me if phantom power feeding back would be an issue but, the BDDI even has the output labeled w/ “phantom power” so it’s likely got circuitry to protect from that being the cause.
Anyway, just brainstorming, maybe somebody else can chime in with ideas about what it could be.