Open your throat and take a little more mouthpiece in your embrosure. That should open your sound up a lot. Also, make blowing hot air helps response, especially on low notes, and can add more edge to your sound. As far as mouthpieces go, a legit mpc is gonna get you a legit sound unless you have very good control of the instrument, so look for a good jazz piece (Meyers on alto, Links on tenor and Yanagisawas on soprano are safe).

There's been a lot of good jazz played on rubber mouthpieces. I appreciated your other response about taking more mouthpice in and etc. Since I started playing jazz and rock while I was still playing in a legit orchestra, I was sometimes admonished for my brash tone and seeming inability to not play with a lot of presence (and volume). I'm going to try taking less mouthpiece in and blowing less air to see if I sound different - course I never play legit anymore! All the rock and jazz sound out of my Mark VI tenor -for over 30 years - came out of a Selmer Soloist C*. Just recently added a rubber Link and a Soloist D to my case however. I think the artist has a lot to do with the sound produced by any mouthpiece. (But I do wonder about metal sometimes). When I was young, metal was looked down on - so I guess I'm prejudiced. Anyway - hope you get a chance to experiment more than I ever have with different mouthpieces.