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I do not know about G&W but check out this thread below:

viewtopic.php?t=7408&sid=fc82454e78781a ... 609c41c2a3

I´ll second cktuba on this one. To play loud, practise loud.
Just imitating the "pop"- sound of funk bass won´t work, though.
It helped me a lot to play as hard as I possibly could, with cracking, piercing, uncontrolled sound (at first).
Huge amounts of air involved. Made sure I filled and emptied my lungs all the way with each note, until exhale and inhale had the same length no matter what the tempo was.
After a while I regained control, but the piercing sound remained. However, I raised the level at which the sound started getting that piercing quality.
Plus, it´s a workout for your abdominal muscles, much better than situps.
After that I did some kind of "warming down" with rochuts taken down two octaves (in the trombone version) and VERY softly.
An overdose of this will kill your lips, though, and might harm the throat, too.

I have been using the G&W Bayamo on my 6/4 Yorkbrunner exclusively. If it is volume and low register certering that you are looking for, you should definately give it a try. Before I had it I played on a Schilke Helleberg II and dug it, but had no idea that a mouthpiece could make such a difference. The sound and feel are spectacular, and it isn't just the steel. The design of the mouthpiece is second to none.

The high register on the Bayamo was, let's say, different at first. It took a few days to get used to. But with the sharp rim and the comfort of the steel, I can now play all over the horn better than I did with any other mouthpiece in the past. Ivan is a great guy and offers a trial period. If you play on a 6/4, you can't go wrong with the Bayamo.

My limits of absolute volume (I'll show you crass!) aren't any higher as far as I can tell, but I seem to be getting more sound at the same effort & tone color with my Bayamo.

As always, there is no guaranteed solution. But, for me, like MellowSmokeMan, I was pretty much done with buying mouthpieces and didn't think that one could be different enough to warrant spending my cash.