Mark IIC & effects loop

[Mulemasta]

Can anyone help me work why the Boss DD3 sounds really distant without any real effect in the IIC loop on the lead channel, it sounds like it's in front of the amp and the repeats have no distortion, however works fine in the loop of a Mark IV without problem? Thanks

 

[Boogiebabies]

Try changing the V2 preamp tube and cleaning the effects loop jacks.