I'm inclined to agree with the boys on this one. But if you really feel the need to change mouthpiece you could try the Warburton range. I play on Warburton mouthpieces for trumpet and piccolo trumpet and I find they really work for me. Perhaps you might want to try the warburton cup equivalent to your Bach 3c and experiment with various backbores first of all (choosing a larger back bore might give you the rounder sound you're after). The only problem with the Warburton range is they're quite expensive in comparison to other mouthpieces due to the fact that they comprise of two seperate parts.
If I were you I'd have a good look on the web at the full Warburton range before making any decisions otherwise your mouthpiece crisis could turn into a financial one!!!
All the best!