I like the Trumcor practice mute. I have a very early one and it works great, even the low register responds well. The newer ones look fancier and have worked well when I try them. Light in weight, too.
City Intonation Inspector - Dallas Texas "Holding the Bordognian Fabric of the Universe together through better pitch, one note at a time."
Practicing results in increased atmospheric CO2 thus causing global warming.
I have an old Humes & Berg practice mute that works on all of the instruments I play. It's a compromise, as it goes in far on the 6/4 CC and sticks out quite a bit on the F, but in general it works fairly well.
I know they are tailored to fit individually sized tubas, but I use my Schlipf mute sized for a 4/4 Nirschl on my F tuba. It doesn't work well with 6/4 instruments, but it's better than the Humes & Berg for my 2 smaller horns.
Steve Lamb
U.S. Coast Guard Band
Rhode Island Philharmonic