The Curry mouthpieces are also a possibility. I recently tried several of them and liked them. Reason I suggest a Geib cup is that you were looking for less depth tha a Warburton and the Geibs qualify. Also you mention the Warburton at 1.3 but then tried a Laskey 28, which is 1.28. The Laskey 30s are 1.3. Currys go up to 1.32 or 1.34 I think and also have a Geib option. Schilke makes a Geib on special order. There are others around. These are a couple of thoughts. I have never liked the conventional cup mouthpieces because they seem better on cylyndrical bore instruments, not conical bore like the tuba.
What lies behind us, and what lies before us are tiny matters compared to what lies within us.
~ Ralph Waldo Emerson ~