I think about this very frequently as well. I think “it depends” is the name of the game in this domain… Are you considering the timbre to be the entire sound field over time, or parts of the sound field? Likely it can be both, or sometimes one or the other.
For instance, a piece I’m working on now uses 3 or 4 pitched tones from a synth patch, often droning onward, or fading in and out, or pulsing rhythmically. The timbral flux comes from various manipulations of some choice parameters on the synth, but actually it’s a fairly simple patch, and most of the stuff is caused by a dense effects-chain that has several interrelating channels. So technically it’s one sound source with 8-10 reactive chains processing it.
Where I currently am with it is thinking about “inspecting” the sound field over time. Kind of like spinning an object around in your hands.But sort of like a kaleidoscope, or a very faceted thing, all the different angles of and nooks found while exploring it cause perceptive change, even if the object itself doesn’t really change much on the whole… it’s about the reciprocal relationship between subject and object, if that makes sense. So things such as dynamics and volume changes, both for each track, and in the signal chain, and panning cause subtle to crucial changes in what is heard. Because of the interrelational parts of the signal flow the sound is able to develop both on macro and micro levels, always correlating.
When reading your post, I immediately thought of Cam Deas, he uses a 12 string guitar as an interface for synthesizer and sonic transformation. I think his work is really stellar.
Here’s a track from the middle of the album, but really you should check out the whole thing, the flow is really great.