" voglio bene a mio marito" is less than " amo mio marito".
"voglio bene a mio marito" sounds a little strange. I could think that the relationship with your husband is not very very passionate and maybe your relationship is became a little weaker than what i expect there is between husband and wife.
The verb "amare" has a very strong meaning, usually it is used for husband/wife/girlfriend/boyfriend or for own children/brothers/sisters. I mean it is used for closer relatives, for very strong relationships. Rarely it can be used for the best friend, just if it is very very important for you.
" voglio bene a ..somebody..." is used for friends or animals and it indicates a very positive relationship with this person.