In this Video Neil shows you how to improve the tone and get a more realistic sound from Guitar Amp Sim Plugins by using Impulse responses of Guitar cabinets,

These are loaded into a convolution reverb plugin and inserted after the Amp Sim in the plugin chain.

In this example Neil uses the Sansamp PSA1 as a preamp, but you can do this with any Guitar Amp Sim Plugin.

Best results are achieved with plugins where you can disable the built in cabinet emulation or model so that the pre amps output is routing directly to the Impulse response.