Boss ODB-3 Volume Problem in Tube Amps

So I've been using a Boss ODB-3 pedal at home on a small combo amp and have no issues with it. Ditto when using it on a Markbass Little Mark III.

The problem is when I tried using the pedal on a tube amp, like an SVT-3PRO or an SVT Classic, as soon as I engage the pedal, there's a considerable volume drop. Even turning the Level knob all the way up doesn't really fix anything, as the overall volume is still slightly lower than if the pedal is turned off.

I tried using batteries and a power adapter but the same problem happens. The signal goes directly from a Music Man with active pickups, to the ODB-3, then to the amp input.

Is there something I'm missing when it comes to using the overdrive pedal with higher output amps? Its as if these amps are limiting the output through the pedal, but I have no idea whats causing it