Some pedals flip (best case) or return signal that is out of phase with the original signal. Say you and I bounce tennis balls off a wall. If we throw and bounce together we are in phase, but if I bounce while you throw we are out of phase.
When such a pedal is doing its own thing this doesn't matter, but when you blend the output back with the original clean you get mud.
I use a crossover and blender to process only highs and leave the lows altogether. Sounds massive.
Team Trace Elliot #1, IWNBARMPB Prezident, Mediocre Bassist #399, Old Basstard #86, PBBBC #2