I can't hear the difference between the preamp emulations

The differences between the emulations are subtle, as indeed are the differences between different preamps in "real life". Perceiving the nuances between the emulations will depend on the source material, the microphones used, the quality of the listening environment, the technical listening skills of the listener. Increasing the harmonics level should make the differences more noticeable, as should listening to two very different preamp replicas.

Listening to the differences of drums over monitors in a bedroom, for example, is less likely to show up differences than listening to a vocal recording on headphones in a studio.