tedtelle1 – What general domain of music are you focusing on? Classical/symphonic/concert band, or jazz/funk/rock/pop/etc?
Like Benjamin said, if you're enjoying the Yamaha mouthpiece, there's nothing wrong at all with sticking to that for a while. If you like that piece and want a less bright sound, the most affordable and effective way to do that would be to try harder reeds. Try a 2.5 or even a 3. You should be able to have a lovely flexible sound with those reeds and a Yamaha custom mouthpiece once you've spent enough time on it.
Alternately, a larger Meyer-like piece might work well. Your Meyer 6's tip opening is really good for lead alto playing and a buzzy bebop sound. You might find more darkness and complexity if you try a 7 or an 8 from Meyer or a comparable tip opening in a Vandoren V16/Java (not jumbo) or D'addario mouthpiece. One of the Mouthpiece Cafe Meyer-like or Soloist-like pieces would be worth trying, too, if you can. A bit more expensive, but not too much more, and made by some of the best people in the business. Tons of good options out there these days!
Regardless, if I were you the first thing I'd do is try a harder reed. It might take some getting used to, but in the end you'll have more strength, flexibility, and control in your sound.