I'm pretty new to nebula, but I spend most of my time working with DI guitar, and this is how I've been doing it. I usually get the best results by experimenting with different programs before the amp sim when I'm in the mixing stage. I'll experiment with everything from Alexb's preamps and CDSM's Vox (producers pack 2) and tonelab (its not listed but you can still get it), to my favourite Henry Olonga programs (his quickmix programs are amazing). To emphasize a favourable character of the amp, using the right combination of preamps will make the ampsim really sing without it sounding digital or forced.
There are tons of great cabs out there. I will use Kalthallen cabs (free), ownhammer, and I still like to use my old redwirez impulses sometimes. I recommend you don't rule out impulses as there are so many great ones, and I don't find most of the nebula programs to sound significantly better (I am hoping new ownhammer cabs will change that). I find impulses are better for customizing a sound by using cab sim programs like mixIR2. Using a program like that can be a godsend when no single cab sounds right.