Something darker than a hard rubber #4 Yani piece? And on a bronze horn? That should already be fairly dark. Anything darker would probably have to be a classical type piece like a Selmer C*. As to straight or curved necks I prefer curved as it works better with a neckstrap which takes the weight off of the right thumb making it easy to play for long periods of time. Alot of players prefer the sound and response of the straight neck. But straight necks don't work very well with a strap and without a strap the weight of the horn on the right thumb can be uncomfortable even painful after playing a while.