You may want to consider Sito mufflers from Italy. I have used them on many of my airheads. Very close to original sound, and long lasting. They have a ceramic coating on the inside that helps stop them from rusting from the inside out. Pricing is around $300 for a pair. Sito is the parent company of Leo Vince, and has been in business since 1954. They stand behind their mufflers pretty well.
Lots of BMW shops carry them, or you can order direct from Leo Vince USA.