In regards to IR's changing the feel of the amp, I say it has to be a matter of perception and I believe it to be completely true. First off, different frequencies from your guitar have different decays when playing. Changing the relationship between the amplitude of different frequency bands will change how you perceive the sound, thanks to psychoacoustics (amplitude- and frequency masking, etc.). Second and lastly, the way you monitor your sound will to some degree also change the dynamics. Some rooms and monitors (and even the reverbs you put after the cab block) have different decay times for different frequencies.