I know someone who has the "Silent Brass" practice mute for trombone, I haven't tried it but I've heard him use it and it works. He's a very strong player and we could barely hear him while he was using this mute.
The only tuba mute I've had experience with is the one made by the students of Tennessee Tech. (
http://orgs.tntech.edu/tuba/mutes.htm)
I understand that these mutes are hand built by the students and sold as a fund raiser for their music department.
Overall fit and finish is at best, poor. They perform well enough though. The metal is cut crooked, the rivets are poorly placed, the wooden end plug first is a bad selection of a poor grade of plywood and secondly looks like the student tried to cut it into a circular (?) shape with a keyhole saw, the plug is nailed into place with cheap poorly place and spaced brads, the handle is a cheap plastic cabinet handle that will come loose in a short period of time. My reason for buying it? I got it on Ebay for what I felt was a better price than what I would have paid had I bought it direct from the school.
The faults that I have mentioned are easily remedied by anyone with half a brain and a little motiation. I'd spent the money and decided to fix it right. First, I replaced all the rivets on the seam, I then removed the plug ad put on a REAL manly type metal handle. I then cut off abuth 1/2" of metal where the brads had secured the plug so I would have fresh metal to evenly mark and drill hloes for the roundheaded screws I would use. I cut a new plug and trued the outer angle and circular shape up by turning it on a lathe, I then remounted and secured the plug. End result, it looks a whole lot better and it still works even though
I worked on it.