I think it should be stressed that there's nothing wrong with "subtleness" as a word, and that it is possible to think of circumstances in which it might be a better fit aesthetically. For example, if you wanted to avoid a possibly twee rhyming sound, you could say "with subtleness and delicacy" rather than "with subtlety and delicacy". Of course, judgments about aesthetics are subjective, but having the two words available does give you the choice of le mot juste, as we say in English. ;-)