I'm going to be a bit provocative here.
I find interesting that some people don't think there is a lack of warmth with the strings, and yet, here they are on this thread saying: "I want a warm, expressive, romantic sound". The way I see it, romantic is synonymous, if not damn close to warmth! If your happy and content with the warmth you already have, don't make a point of it saying that you want that!
Hmm.. that is provocative. [[;)]]
I would argue though that the description 'warmth', generically used, is insufficient of itself to define the 'romantic' sound. You can have warm strings without evoking anything remotely 'romantic'. A more careful definition is needed, I feel... Generally there is a fluidity in the phrasing that goes a long way to making the sound, a sense of 'movement' even in individual notes... On the whole it should have a light and airy touch, though ideally there would also be samples that would allow one to 'dig in' more.
More thoughts?