Re: Perhaps the best Dumble tone? (my .02)
I've in the past used my Tweed Blues Jr with a T-Rex Replica up front as last pedal in line and had no real bad issues, though I use it for subtle effect. I think other pedals before it can hurt it or not, etc, ..experiment a bit I guess.
Here's and old clip/noodle of mine with Blues Jr with Replica up front: http://www.youtube.com/watch?v=3FMx3ISyqtM
EDIT: I believe the more you use Distortion, the more the problems exist with delays up front, but others can be more helpful on site.