Yes, in a way the metaphone is like the sordine for the violin, a device to alter the timbre but not to add expression.
In this example it even diminishes a bit the expression.
On some instruments the change in timbre might have been too big to make a progressive version of the device.
On the other hand..
I compared the sound of my Schiedmayer to a Mustel of the same period. Just as you have stated in the past,
the sound of some registers are quite a bit sharper than Mustel. The biggest difference are the harpes eoliennes. If I use them on my
instrument with metaphones, they sound much like the ones on Mustel WITHOUT metaphone.
although the effect of the metaphone is huge in my case, I did some test on my instrument playing with one hand and controlling the
metaphone-slider gradually with the other, ..the results are fantastic...
Not every register changes as much as another by the metaphones, I think it's incredible beautiful to make a crescendo and
to hear some dynamic timbral balance change in the sound while doing so. Just like the horns who turn to "cuivré" in an orchestral context
of course this is already present in some degree, but still..
I wonder how it would have worked if I could have controlled the metaphones with the knee levers...