AN IR can capture all sorts of thing besides the cab including the amp and EQ.but changing impulse responses (which are indistinguishable from a real cab being recorded) makes a huge difference.
It's also possible your and my interperetation of huge are very different. If i can make it with a little bit of eq knob turning it isn;t huge in my opinion.
Playing live yes, but if you are trying to duplicate the tone from your favourite recording there is a strong chance it was miced near field and that is what you are hearing in the recordingWe listen to speakers in the far field vs. near-field with one ear only an inch or so away from the speaker.