Truth here ... I reckon I have fairly good ears. Most of the time I can't really discern much difference between distortion and stacked distortion. Add in drums and a loud and distorted guitar, and you can hear what? You have a valve amp, already breaking up, you throw in a dirt pedal, pushing it further and adding more compression and some (odd or even order) harmonic content. But everything is still going through the the same output valves and transformer (and possibly the same valve rectifier). Add another dirt pedal and you are in the land of diminishing bass fx returns (ha).