I don’t know if I misunderstood what you are saying, but what I get is you mean that basically in any speaker capable of reproducing the frequencies in a note, instruments sound exactly the same. This can’t be, because it is not my experience. Is this truly your experience??
How can the differences be attributed to tone if the same DAC, cables, amp is receiving the same string of data, which is then converted to electric impulses and then reproduced as a group of frequencies ( a note)?
Why two speakers capable of reproducing those frequencies sound different? Do you agree they can and usually sound different?
You are trying to describe a sound characteristic that can’t be measured, but I’m afraid all you can hear in sound, including timbre, falls within the measurable domains of amplitude, phase, and frequency. The reason you hear differences is because those measurable attributes can vary at the transducer and always bounce around (or not) in your listening area, affecting the perceived FR, amplitude, and phase at the listening position. So the differences you hear lie not in some made-up immeasurable quality of sound, but in the immense complications of variations in transducer FR, directivity, and room characteristics.
Differences in timbre arise from different frequency response and amplitude - the relative volume of different harmonics and attack and decay amplitudes. So the speakers themselves might sound different based on measurable frequency response attributes of the speaker. But even if the speakers have identical (on-axis) FR, the speakers then interact with the room. If one has wider or narrower directivity at a specific frequency, the room configuration may cause a frequency to be (de)emphasized, changing the perceived timbre at the listening position even if your speakers have identical on-axis response. Which is one reason we look at directivity/off-axis response of speakers. Also, if you are comparing speakers side by side they are in different positions within the room, and the room interaction will be different, changing the measurable values at the listening position
even if the speakers were identical with respect to both FR and directivity.
Which is why the philosophy here is to start with linear speakers with uniform directivity (and linearity+low distortion in everything prior to the speaker terminals, but that is now cheap and easy). Linear on-axis response, uniform directivity, and low distortion, at the speaker give you the best chance - and it’s only a chance because the room is such a big deal - of fidelity at the listening position. This is also an approach with a substantial grounding in audio science. That’s why you are at Audio Science Review.