Pardon me to jump in but I see things a bit differently in some points. Not an expert, just my opinion.
If you don't break the 1/8 impedance ratio I think nothing will happen while you increase source output impedance.
If by increase you mean breaking the 1/8 impedance matching ratio I think that the bass and mids will be susceptible to impedance mismatching effects (below) rather than detect an increase in mids and treble.
Pretty sure that's not accurate. Again, if by increase you meant breaking the 1/8 rule I think impedance mismatch can still produce some of these efects in various proportions and intensities due to reduced damping factor: roll-offs and tonality alteration towards source impedance response curve (for instance if the source has an L-shaped impedance response curve the phones will get darker and so on). Also, apart from FR curve alteration - clipping (reduced dynamic range) and distortion can occur in severe cases of impedance mismatch.
Cheers.
Edit: also, afaik cable impedance counts towards source impedance so have that in mind as effects on tonality (that can be measured) can vary from none to many depending on all parties involved impedance and the breaking or honoring of 1/8 rule. Not talking about non-measurable effects (resolution, dynamics, soundstage, imaging etc.) - that's everyone's guess.