The Goldbeck may be the darkest. I have one here with a repair horn and it is not too bad. It needs to be refaced as it is pretty rough. If you are interested, I will see what the guy wants for it. Somehow it seems more centered than the Goldbeck alto I have.
I think the Link STM is pretty dark as metal goes.
Try a Vandoren HR and you may find what you are looking for.
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