OK, with a series loop the amp can compress the wet and dry signal together depending on how much gain you use and how much power tube saturation takes place. The more compression that occurs, the less range will be audible between dry and wet on the mix knob. One way to overcome this is to use another amp for the wet effects, ie a wet/dry setup.
In your setup now, as long as it sounds the way you want there's no need to worry about how low the mix knob values are.
