Looks like he match EQ'd the guitar rig Preamp to an engl preamp, then put a cab impulse after it.
You could play a sine sweep with a guitar rig head without a cab on it and get that snapshot.
Send a sine sweep into an amp head and out of the send, or preamp out back into your interface and take a snapshot of that.
Force the guitar rig to sound like the actual amp with match EQ. Export the sine sweep and turn it into an impulse.
That's actually pretty clever.
Basically forcing the fizzy Guitar Rig preamp to sound like the fizzy Engl preamp.
(Not fizzy as in bad, but as in all preamps by themselves sound like that)