I've only had experience with genuine tubes and the helfands tubes. The helfands ones work great with the genuine crowns; very good quality in my opinion. And of course, you just can't beat genuine. Nice, smooth precision with the threads. The crown glides perfectly when threading it to the tube, and is very reassuring; not much worry about cross threading (whereas the rep tubes always leave me worrying).
Actually, only gen crowns will fit gen tubes and some aftermarket tubes. The inside diameter of the gen crown is much, much larger than a rep crown's; as such the tube has a larger mating thread for the crown.
I buy my crowns in bulk from my supplier- usually in batches of 10 to 12 at a time. I need to purchase more... I think I'm currently all out (it's been a very busy past few months).
Well, you'll want to simply use the rubber gaskets that come with the tube. There's a total of 3 on the tube, and one in the crown. The gaskets will do the job of sealing. For securing the tube to the case, a very small dab of threadlock like loctite will do the job.