Don't downplay the importance, in harmony singing, of a strong voice to sing the TUNE! I've heard three strong, instinctive harmony singers singing together - and none of them singing the actual melody. You could more or less work out the tune by the gaps where none of them were singing, but if your harmonies aren't particularily interesting (mine are terrible), perhaps you should concentrate on being the strong melody person who makes the harmonies possible. It's a very important job, even if it's not as 'glamourous' as singing the actual harmonies. Someone has to do it!

Andrew