I did make the noise part in my Yocto 808 plugable, so the transistor and OpAmp is changeable,
also i have various means of connecting small capacitors to reachable pins, to play around with the sound.
Granted I often dont use the best of studio monitors to listen to the results, which maybe alters my comment useless
since with electronic music it also depends on the speaker you listen with
But I use to alter listen and monitor audio waveforms on a 100 MHz oscilloscope, so I can be pretty sure about if and what I alter.
And the noise transistor doesnt make a noticeable distinction in my case, I tried plugin in jelly bean parts like the 2N3609 ( notice the different direction of base emitter collector so you have to bend the pins in order to plug them in )
soundwise the result was pretty much the same.
Although the more I think of it, I maybe have tested with a wrong supply voltage
But anyway, I like to add an electric engineers opinion (that would be myself) on such subjective stuff as rare transistors and how they alter the sound.
To put it simple, it simply doesnt, it wont automatically render your music better or worse, if you use a "wrong" transistor, as long as the current and voltage ratings fit you simply can try anything.
In the TB 303 there are also some "rare" finds, simply JFET in the output section, driving the diode ladder filter, this one also doesnt alter the sound to an extent one would suspect once reading thrugh different online sources.
But to put something positive at the end, it is always nice that there are still some people out there interested in the analog process of making music.
cheers