That's kinda the nature of the switch.
Your increasing the gain a bunch in Tude mode and trying to dirty the amp up. With an increase in gain you get an increase in volume, nature of the beast.
However....
Try removing R8 (this is the 2K7 resistor in parallel with the 0.47uF cap).
This resistor when in Tude mode lowers the cathode resistance to a lower value than in Tweed mode. If you remove it you might get the volume levelling out a bit more. You might loose a bit of gain though, so keep that in mind.
You don't have to totally remove the resistor btw. Just desolder or snip one end of the resistor.
Chris...