Do you have any scientific proof that timbre is dependent on the FR. Or is this another one of Inks' assumptions? Seriously though, would you mind enlightening us on what frequencies are specifically impacted and how they impact timbre? If not, you have as much information on the subject as he does.
Other than phase, FR and the time domain together hold all of the information of the the sound wave. Considering that earphones have much shorter decay times than actual instruments and that it is far harder to detect these time domain variations, the major difference is made by FR.