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Just get a MW-25....everyone seems to say it's tone is too dark.

Although I have never played a Mirafone, I have heard from many people that they have a brighter tone than that of a Meinl Weston. That could be your problem right there...
I have been complemented on having a dark sound while playing on a Yamaha 641 BBb with a Bach Megatone 18. I think mouthpices with more mass can give you a darker sound- try a Megatoni (I think that is what they are called). I too use the "frown" embouchure, but I think that the best advice comes from T4S's sig.

I highly recommend getting a copy of "Song and Wind." It has a lot of valuable information as well as pictures that reinforce the topics being discussed. There are several pictures that show various embouchures that may be of assistance. The Berp you are waiting on is an awesome tool, as is a visualizer. Put all these together, along with a private lesson or two, and you've got a good starting place. Be patient with yourself and keep a journal of your practice sessions and performances. Annotate your successes and failures and take these to your lessons. There will be a moment that everything comes together and you will achieve the sound in your head. That will be the moment upon which to build the rest of your playing career. Keep striving to improve.

Get yourself to a good teacher. There are many reasons that your sound is bright. I usually find a certain kind of sound that I call "strident" as in annoying, in students who had no formal experience with a teacher and these students were all buzzing THE LOWER LIP to produce the sound. I have to wonder if this may be your issue and you need a good teacher to help you over this. Lessons by e-mail just aren't going to cut it.

Just my $0.02.

For one to play with a dark sound, one must be thinking of a dark tone. Listen to some other instruments with a dark quality. I'd recommend the Yo-Yo Ma Cello suites on RCA. Gene Pokorny always sounds great too. I'd lean away from blaming the horn, although mouthpieces do make a difference.

Just think "How would Batman or Darth Vader play this lick?"

MellowSmokeMan wrote:Just think "How would Batman or Darth Vader play this lick?"

That's the best advice I've ever seen here!!!

Of course, ditto what has already been said about the value of online lessons.

In my own personal experience, what helped me darken my sound was focusing time on mouthpiece buzzing. In particular, I worked to get a "thicker" (anybody have a better word for that) buzz. (Thin buzz = the sound of maybe a forty or fifty bees / thick buzz = the sound of maybe a few hundred bees.)

It's an easier concept to demonstrate than it is to describe.

Whereas I do think that equipment can have a factor in the overall sound a tuba player makes, I think a lot of it has to do with what the player is doing.

When thinking about the things I do to produce a dark sound, I think one of the often forgotten elements to playing is to use the warmest air possible. Often times we get ingulfed in worrying what our mouth looks like or where we're placing our tongue, or where we're breathing from. That all is good and well, but if you're using the wrong air stream, you can still sound like a goose on steroids.

You might possibly be using too fast of a stream of air. You may need to work on using more volume of air, and less speed. You can push the same amount of air through your instrument at a slow speed as you can if you push it through very quickly, maybe even more so. Also, check to see that you are using the warmest air possible. An easy tester for this is to blow onto the side of the bell like you would be if you were playing the instrument? Does it fog up? Change things until it does. That fog will let you know it's working, and condensation will probably form in the horn quite a bit if you use hot enough air (mine does).

Anyhow, that's my $2.64 worth. GOOD LUCK!