Most all IRs are unique due to the many variables involved in creating them. While we generally think of them by the speaker used the microphone, cabinet, power amp and even the room where created have large impacts on the sound.
We all want the best guitar tone, whether it be for blues, country, pop or rock and impulse responses can help us get there.
With the wide variety of guitar amp modeling software such as the S Gear, Softube Amps, Peavey Revalver, Positive Grid Bias, Amplitube and hardware such as Line 6 Helix, Atomic Amps Amplifire, Fractal Audio Axe Fx II and Headrush the door is open to professional sounds right at home.
First thing to note, do you have experience with specific speaker models? No? What do your favorite artists use? This is a good place to start.
Then, what style of cabinet is best for you? Open back smaller cabs sound, well, smaller, have less lows and more focus in the mids.
Now, I recommend starting your IR discovery with a new preset with bass, mids and treble all at noon. It can produce a preconceived bias to load an IR into an existing preset and expect the sound to automatically be better. Of course that preset was setup for the variables present at the time of making. Also, when use to one IR then switching to another can cause a whiplash to the mind. Even if an IR sounds off at first it is good to take a few minutes to adjust to it before making a final decision.
It is important to find an IR you like the basic sound of but maybe sounds a little too thin or a bit too bright. Start with a setting a high pass filter at about 80hz and low pass filter around 8k hz. This will bring the frequency response into the range of the tones we hear on a CD or Mp3. Next step is to adjust the amp bass, middle and treble to see if you can balance it our to your liking. If not, load an EQ after the amp either before or after the cabinet. EQ before the amp is more for tailoring how the amp distortion reacts and will have less of an effect on the overall EQ vs after the amp. I prefer parametric EQ but graphic EQs tend to be easier for those starting out.
With standard tuned guitars I find EQ adjustments around 100-150hz are good for lows, 400-600hz for mids and 5k-6k hz for highs a good place to start.
We welcome and questions or additions to the discussion in the comments.