If you are going to "soundproof" a room of your house, (I put that in quotes because it is pretty difficult to actually do this in a cost-efective manner) hang up some carpet and heavy blankets on the walls. If there is a little space, say 1/2", under the door, more sound will pass through that space than the entire rest of the door. Putting something in that crack will help, and hanging soft and heavy stuff on the door will also help.
A big problem a lot of people have with casual acoustical treatments is isolation of vibration. When you think about it, this is all that sound waves really are: vibrations in air. If you can stop the vibrations, you can stop the sound. This is very important especially where tubas are concerned, because the low frequency notes that we play produce a lot of vibration that can be very difficult to dampen.
The reason why hung ceilings are so prolific in many classrooms and schools around the country is that they can be pretty effective at damping vibration for several reasons: They create an airspace between the ceiling and the floor above, and the material actually absorbs some sound waves itself. So if you are really committed to soundproofing, this could be an option, especially in a basement area.
Also, don't discount the effect that the heating system and ductwork in your house has on the distrubtution of sound. Plug up any vents in the room in which you are practicing and you should hear a big difference in the level of sound that is escaping.
Good luck man, and I hope that you can get your problem figured out! I sure wouldnt want to practice for any length of time with a practice mute in if I had a choice in the matter.