Okay, so I have my FX send connected by a standard guitar cable into Input 8 on my soundcard. Remember - I have NOT unplugged the speaker from my amplifier. I can still hear the sound of the amp coming through the cabinet - just very quietly as my volume is down. That channel is selected in Logic and I've added an IR to the channel in Space Designer - you should use whatever IR host you have on your computer. If you don't have one check out the links at the end of the tutorial for some free hosts!
At first I've bypassed the IR host and you can hear that the tone direct from the amp is pretty shocking. In actual fact, it's an awesome tone, it's just not running through any cabinet yet and the EQ of a guitar cabinet is very specific and adds a great deal of body and resonance to our sound. Therefore, listen to what happens when I run the signal back through the IR host to regain the sound of the Power Amp and Cabinet.
It's quite remarkable the difference the IR makes to the sound. Remember, this is still the tone of MY amp, there's no preamp modelling going on here at all. The heart of the sound is coming from my amp and I'm getting that tone without micing anything up at all. Another thing I should point out here is that my amp is really, really quiet in the video above. You could have a conversation and drown out the sound. We don't need to turn up because we have no need to drive the power section of the amp. The resonance and tone that cranking the amp volume up would produce is being modelled for us by the IR.
Now all the plugins in your DAW such as high quality delays, convolution reverbs, modulation effects, compressions and EQ's that you use for mixing are available direct from your amp! I know a few people who do this live. Just an amp head plugged into a speak load, laptop and monitor speakers, using their DAW to produce all their effects. Very cool!