Hey,
So I'm giving pops a try (POPADS), and I'm wondering what are the most crucial things to be split-testing.
I've set up campaigns and currently I am split-testing performance in popunders, pop-ups, tab-unders.
I've also selected categories that I feel relate to the offer being promoted.
I'm running sweeps. Should I be testing LP's as well as DL?
Do sweeps generally NEED an LP? If they do, I'd imagine the angles must be dependent upon the category selected.
Oh..and should I be choosing Desktop or mobile for each campaign? Or just test both (and all) in a single one?
Thank you 
You can split test a lot of things there, but there's a hierarchy that many users follow and it's up to you to do so because there's no "unique" way to do it.
Major things you should consider in the first stage of your campaign:
1. Mobile vs Desktop. Actually, separate the campaigns because the bid prices are different.
2. OSes
3. Browsers.
4. Landing pages - Yes, you need landers for sweeps 99% of the time on pop traffic.
5. Offers - This is more important than ever in the first stage. Don't forget it.
After that, you can start split testing things like popups vs tabunders, devices, brands, browser version, etc.