First of all: dimensions.
The ST-171 cup mute is the right cup for large bore TENOR trombones (8.5 bells). It covers almost all the bell. The smaller one is suitable for small bore tenor trombones.
Response.
The mute is well balanced and really in tune in all the registers.
Sound.
The sound is AMAZING. It is a really peculiar mute....the material it is made (a mix of cardboard, wood and laquer) creates a UNIQUE sound, that is really a lot close to the bog band solos from the 20s.
It is really nice to play, and can be used equally in jazz music and in classical music (bear in mind that the cup is fixed).
Personally I tried to muff it with different material inside, you can just try out new timbres. Even adding a soft plastic or gum tape on the border of the cup may change big time the sound in recordings. Take your time!
Quality.
The construction quality is nice, just take care of your mute because dropping it a lot of times may scratch the laquer (that is really thick and may come out in big pieces)
NOTE:
The cup is NOT adjustable! Fixed cup.
