Well, both the UA5 and AD20 are pres+A/D converters in one, so running either behind the JK Labs box you'll be going through 2 gain stages.
Anytime you go through 2 boxes, you will go through two gain stages. It doesn't matter if the box is labelled a preamp or an A/D, either will still have pretty much the same basic design of a gain stage, the preamp box will just allow for greater gain. I and others have covered this concept very thoroughly numerous times, do a search for more info.
So it really doesn't matter that the UA5 and AD20 are pres+A/D's in one. The bigger issue is how much minimum gain you will have from the two boxes you're using and whether that will be too much for your needs. I've found I've usually needed about 30db+/-5db of gain total. The AD20 has a minimum gain of 17db IIRC, so hopefully the DVC would have a gain setting of 10db or less at a minimum, otherwise I wouldn't use it with the AD20. The UA5 allows for 0db gain I believe, so that would be fine.
I would agree with Brian's ranking, but I don't know if I'd agree with the parenthetical comment about DVC with no preamp for the first two options. You will need to go through an additional gain stage no matter what if you use the UA5 (or any external box), but many gain stages sound better at low levels of gain and not as good at higher levels of gain. It may be that the DVC has a better sounding gain structure than the UA5, in which case, I'd choose DVC (with preamp)>UA5 over DVC (no gain)>UA5. (I haven't heard enough of the DVC box to comment, but based on the quality of Jon's work, I'd bet that the gain in the DVC sounds better than the UA5 or esp the AD20.)
Of course, all this is assuming you're talking about one of the JKLabs DVC boxes that allows adjustable gain starting at 0db. I'm not sure of his design, but I'd imagine that even at 0db gain in an adjustable DVC, you'd be going through the gain stage. So you'd have two total gain stages if you went DVC>UA5 (whether or not you had the DVC set to 0db gain). If you knew you'd never want gain from the jklabs box (as I did), you could get the ECMS-22 box that has no gain stage. Run the ECMS-22 into the UA5 (or V3, or MME, or AD20) and you'd only have one total gain stage.