I remember the first time we dropped a JFET in place of a BJT. Wow! What difference. More tube-like, softer, rounder, less detail.
Interesting for a couple of reasons. First, the idea that you can simply substitute one device for another without any other changes is always fascinating. Second, the two types of silicon are radically different, yet the measured results are nearly identical.
JFETs are field effect transistors, while BJTs are junction devices. What this means is complicated and beyond the scope of Paul's Post. But in simple form, a JFET is more of a voltage device while a BJT is more a current device. Tubes act very much closer to JFETs than BJTs do.
And, interestingly enough, JFETs sound closer to tubes than BJTs.
Problem with using JFETs is their softness and tube-like characteristics are sometimes so extreme as to lose detail, sounding dull and wimpy. Which is why we rarely resort to them and use MOSFETs instead. (though a JFET can be used to tame an aggressive BJT circuit for good synergy).
The point of this post is this. Every component in the circuit has a sound to it.
The designer's challenge is to know where and when to use which ingredients to bring the music to a point they want.
It's called skill and experience.