I think 'ti voglio bene' is more of "I like you " or " I am fond of you ", it could yet refer to love as well ,but not as romantic as l'amore in ' ti amo'.

/ti voglio bene/ and /ti amo/ may both be used with nearby meanings in romantic contexts; -- inside the family, (obviously excluding wife and husband) only /ti voglio bene/ is used; the same rule holds between friends; --- the literal meaning of /ti voglio bene/ is : /I want you to be {happy, fine, etc.}/ I think it is a marvelous expression (if the feeling is authentic, of course!), as it does not imply the desire of possession that romantic love might imply; in so-to-say everyday usage they carry exactly the same meanings; if you would like to explicitly stress the difference, the variation /voglio il tuo bene/ is available.