treble extension / bass deffinition ??

The other day i was demonstration (very effectivly) on a cheap hi-fi system that treble extension was needed for bass definition. If i wound the treble control right down, all the bass/precusion merged into a muddled mess. Extra treble gave the bass and precusion the impact and clarity that i like. The system was a marantz CD63 KI sig, hitachi 5300 70's amp and cheap jamo speakers.

Question: was my test flawed because the phase shift introduced by the treble control was also effecting the bass definition, rather than the lack of treble extension alone?