There is one particular IR that really helps with the mellow lead tone; it's the Eminence Texas Heat speaker, available from their website. I wound up using the real speaker in a bunch of my amps, because it takes the fizz / ice pick highs out and leaves you with just tone and sustain. The IR sounds exactly like it. Most IRs I fiddle with sound different to, but not always better than the cabs. This one seems distinct. I always come back to it.
Hook that to the Litigator amp and put a boost / OD in front of it to taste and you're into that darker, Derek Trucks sort of wooly tone.
