You're right. And both halves of V2 are only used in the OD path, so with Blend on 0 V2 is completely out of the picture and should not have any effect on the output volume. My bad there.More precisely, both channels go through V1b, but V1a is solely for the vintage channel. V2a and V2b is solely for the OD channel.
I didn't have a 5751 to sample unfortunately (expensive suckers these days!) but tried what I had in my box of old tubes which were 12AT7, 12AY7 and 12AU7 which are all lower gain. There was definitely a drop in volume but I'm probably remembering that with the blend set somewhere above 0. I ended up sticking the 12AX7 back in there.

