You've hit upon a true conundrum. It's like describing what an orange tastes like - without ever having had a lemon or a grapefruit. Listen to recordings of the players whose sound you wish to emulate. Listen as much as you can. Get that sound in your head and when it becomes a part of you it will become easier to reproduce it in your own playing. You can mellow the 43 with a deeper mouthpiece, or a trumpet/flugel piece like the Curry TF, or by stuffing a sock in the bell, but ultimately you are the one who makes the sound.