I've spent a considerable amount of time searching for the "dark" "smooth" actually "muffled" sound. Maybe it's my ears but I hate the highs... anything remotely buzzy. I've bought various mouthpieces just to try them and then just resold them on Ebay. I'm also an Engineer so I approached it from the aspect of looking at major differences. For example a high baffle, small chamber, metal mouthpiece is one extreme and a low baffle large chamber hard rubber is the other. There's tons of stuff in between, but I was looking for the big affects.
Metal is out. Just too metallic, buzzy! If anyone says they have found a metal mouthpiece that gives them a dark, non buzzy, tone, I'd be surprised.
I though the Jody Jazz HR Custom #5 Dark was going to be the answer. It wasn't. I sold it. Granted it was pretty good, but very free flowing (#5, .085 gap) and louder than I care for, and still allowed the highs to come through.
Also, the reed makes a huge difference. In general, a thinner reed adds more highs and buzz and a thicker one just makes it harder to blow. I gave up on cane some time ago, just too variable and required changing like underwear. I'm guessing if I go to a thicker reed the sound will be even darker, but I'm not willing to go there; trad-offs. Just because I can play it doesn't mean I want to.
So far the magic combination seems to be a Yamaha Custom 5CM Hard Rubber (Not to be confused with the Yamaha 5C, 6C, etc. which are ebonite/plastic) with a Leger 2.25 American or French cut. In addition to hard rubber, the 5CM has a larger chamber than your typical "I got it with my sax" mouthpiece. The minute I put it on my sax it was an obvious difference in tone; deeper, darker. The hard rubber and the large chamber is similar to the Jody, but Jody has a shorter facing length. The #5CM (.071 gap?) with the longer facing makes it easy to blow and promotes a lose embouchure; I don't need a ton of air volume and every breath last longer without it feeling overly restrictive.
How Leger came up with 2.25 exactly I'm not sure, but it does seem to be some kind of sweet spot.
And the 5CM is like $60-90 (lot cheaper than a Jody!), so if you don't like it, you're not going to loose a lot of money. The Leger reeds are expensive but they last a long time. I've tried other synthetic reeds and they do not respond as well across the entire range.
***Your ear mileage may vary...To qualify this, I'm a hobby player and don't need to overpower a band. But I think staying with hard rubber, large chamber, low baffle (no high baffle) and long facing, gets you the tone, while adding increasing gap and maybe reed thickness, will get you more volume if you need it. The price you pay is more breaths. Trying a 6 or 7CM won't set you back like some of these $300+ mouthpieces.