Re: alto mouthpiece
Its not just size of the chamber, its shape of the chamber and baffle that makes a big difference. I believe that the soloist has a horseshoe chamber and a small rollover baffle. The rounder the chamber, generally, the darker the sound and different baffles add different degrees of edge to the sound. I use a Selmer Larry Teal and I get a very dark, pure sound. Its got a medium chamber, completely round, and no baffle. I'd recommend it. I've got a modern one, but if you can find a vintage model (with the scroll shank) for a good price, go for that. Its a little more focused.
I do not know your playing ability, so I could be wrong, but don't jump the the conclusion that its the mouthpiece's fault. The Larry Teal is the darkest Selmer piece I've played, but all of the Selmer's have the ability to have a very rich sound. I'd recommend spending a serious amount of time working on your tone on your current mouthpiece before coming to the conclusion that you want to move on.