The difference between real tube-circuit hardware and any of the emulations or plug ins, IMO, is that the real tube stuff typically sounds cleaner and more hi-fi, which is funny. The real sound of tubes is usually less "tubey" than the copy-cats, because the plugs are trying to create audible harmonic distiortion and dirt. Genuine high-quality tube gear is trying to be high-fidelity, and detailed with plenty of headroom. It also can't help imparting a subtle harmonic or euphonic warmth, which enhances musicality.
The thing I think that makes the MOST audible difference in wheter something is warm or not is whether it uses transformers, or is transformerless. That's why a lot of my favorite and most musical-sounding gear is transformer-based solid-state, like Neve, API, etc.
