Timbre is relative to the materials and construction of a musical instrument.
An E-flat played on a grand piano will sound different than that of the same E-flat played on a Violin.
Same note, but reproduced on a differently engineered stringed instrument.
Hence...Timbre.
It really has to do with construction of the component in question.
Issues of resonance, decay, and what's called spurous and sympathatic anomolies, can and do, occur.
All materials and design characteristics, within an instrument or loudspeaker WILL alter the sound to a certain, audible, degree.
The goal of timbre matching a center speaker is directly related to being able to produce the same sound, within the mid-range specifically, that is being produced from the front L&R speakers.
And what that involves is having a center speaker that is derived from or engineered to compliment whatever front main speakers your using.
The idea being that the left and right direction of the sound will match seamlessly with your Right and Left speakers when the sound goes from say...Right to Center to Left speaker.
Meaning, a good center channel will, in effect, be able to "pan" an image across the front "soundstage" and also, have been engineered well enough to handle most dialog that happens to occur in the mid-bands, with realtive ease.
In my experience, when trying to build a Home Theater, you should try to get a center speaker that has been specifically engineered for the main R&L speakers, .... or if the vintageness of your gear doesn't allow, then to find a 3rd speaker, that matches your main speakers "Identically", to use as a center.
That seems to produce the best results.
Have fun.
Regards,
John
P.S. For what it's worth,...... Missed the Easyrider rodeo this year but looking forward to the local poker run this weekend.
Here's to hoping I get a winning hand.