I'm not saying light spectrum doesn't help get you dense nugs bro because it does. You need both red and blue spectrums though (some argue that yellow and green play a role during flower as well). This is why I'm thinking of throwing my MH bulb in with a HPS for the last two weeks of flower. Getting the best of both worlds will increase the chances of having some really dense buds. How far you keep the light from your plants is another factor. Light intensity gives buds density (stole that from someone online lol). Genetics is another. Some strains just weren't mean to produce dense nugs. This too is argued all over the place but I've seen it first hand...
My friend grows 3 or 4 different strains all in the same room. He treats them all the same (feeding/trim/lights/etc...). His C99 plants produced the tightest and most dense nugs I've ever seen, while his bubble gum was lighter and fluffy. The only factor that could have made the difference was genetics. Since you're using mystery seeds you won't know how dense your buds will be until you get to the late flowering weeks.
Not sure if this was any help. I'm pretty baked lol I picked up some green crack and lemon haze last night. This green crack is no joke. Really dense buds with a crazy cerebral high. I took 3 hits off my bowl and set it down. A few minutes later I took a 4th hit... that's when it got real intense. Best smoke I've had in a while.