When I was young, things were different. You chose your particular instrument, and that was that.
But in the last thirty years, not only have the number of mouthpieces and mouthpiece makers exploded, people now can buy aftermarket clarinet bells and barrels and bass clarinet and saxophone necks to "improve" whatever equipment they already have.
I can't make any pronouncements about bass clarinet bells or bass clarinet and saxophone necks because I've always used what I've had. However, I'm fairly confident that the process of choosing an aftermarket component holds for those also. So here are two guidelines:
Don't buy something outright (unless you can return it) on the basis of others' experiences.
A component that works extremely well for one person may not work well for you, even if others have the same make and model of instrument that you have.
When the Backun clarinet bell came out, everyone was crazy to try them. At ClarinetFest in Maryland, the principal clarinetist of the Richmond Symphony tried one one with me and another pro player listening. The sound improvement on his Yamaha CSG clarinet was astounding.
Hoping for the same results on my Buffet R13, I bought a used one from a friend in D.C. without testing it. Guess what! No difference in sound!
I later had one of the accomplished players in Blacksburg try the bell on her Buffet clarinet, just for fun. Improvement! So I sold it to her and she still plays it many years later.
Try before you buy. Don't ever take anyone's word for it, and especially don't buy based on makers' description. Try it on the actual instrument you'd use it on.
Especially regarding mouthpieces, barrels, and bells, don't buy only one example, buy three to try. (If there is no return policy, walk away.)
This is much more important with custom equipment made by small makers. Components made by large manufacturers using machines are much more consistent between individual examples than custom-made pieces.
That's about it. Well, maybe not.
No mouthpiece, barrel, bell, or neck will transform your sound or playing ability from mediocre to outstanding, they will only improve it slightly.
If you're looking for improvements by changing components, start at the top of the instrument and go down.
Mouthpieces influence things the most, then barrels or necks, then the bell. Broken down, in my estimation,
70% is your ability as a player
10% is the mouthpiece
5% is the barrel
10% is the clarinet
5% is the bell
Bottom line: If you're only a mediocre player, fancy instruments and custom components will only make your playing slightly less mediocre.
No part of your setup will magically transform things!