[jtkung]

Impulse Responses (IRs) are files which attempt to capture the sound effect of a speaker and cabinet (as well as aspects of the signal chain like mics and mic pres).  These have a profound effect on the sound of an amp -- esp from hardware that attempts to model amplifiers. Cab IRs are an essential part of the sound.

There a huge number of free IRs available. Just Google Cab IRs. For commercially available IRs, there are many providers, but these are the prominent ones to come to mind:

• Redwirez
  
• Ownhammer
  
• Rosen
  
• Fractal Audio (custom ones for the Axe-FX)
  

Since there are often times so many to choose from, here is one way to sift through them even before listening to them:

• Find a guitar tone that you like -- either from a record, favorite artist, etc.
  
• Research the song or artist and see what was done in the recording chain. As an example, you can find info on John Petrucci's gear and recording techniques on the Internet.  He likes to use Mesa Boogie IIC+, Mark V, and his JP-2C amps using Mesa cabs and V30 speakers. With SM57 and MD421 mics -- as a mix, typically with the SM57 higher in the mix.
  
• Say you like the sound of this combo -- then you use this criteria to screen IRs.
  
• I have found that many people like to use the following 3 mics:
  

Shure SM57
Sennheiser MD421
Royer R121
Neumann U47 or U87

And for guys like Alex Lifeson and John Petrucci, the mic is slightly off the cap with the mic almost touching the grille.  

This is just a general guideline for screening, but I found it works well. Also, certain amps pair well with certain speakers and cabinets. The same will be true for IRs, so this is yet another screening technique.  For example, the V30 speaker is great paired with a heavy rock/metal sound. The Celestion Alnico Blue would not be as good a choice, but would work extremely well with an AC30-type tone.  Look for real world combinations which are known to work, and these will also work well in the modeling world.