Man, what a convoluted circuit.
Tone control meets the reverb return.
Raising the reverb control full on should be a higher resistance to ground signal wise than when it is turned down.
(10K + 50K)
I would try disconnecting the reverb return wire from the pan (they are oriented correctly, right?)
If the volume drop at full reverb on diminishes, then the reverb return coil is what may be lowering the volume.
I have included the Fender DR schematic as a circuit reference.
And a copy of the 6650 as I cannot rotate the one that you linked.