I think what you should do is go to a retailer that sells the various units and really look at them. Sure you can look at the dimensions on the specs, but IMO, something that seems pretty damn small on paper looks a helluva lot bigger in your hand.
I only do open shows myself, but as a former PMD660 owner, I can say that while yes it is small enough to be "stealthable" it is still not exactly what I would call "stealthy". I mean yeah, if stealth to you means something you can carry in a coat or your wife's purse, you have a lot of options. But if stealth to you means something that you can put in your pants pockets, sorry, the 660 is one of the smaller "all-in-one" boxes with phantom power, and even being on the small end of the range, it still really just does not quite cut it for pocket size. I mean it's not even close to a JB3. It's really not even *that* much smaller than the UA-5 you already have. Not exactly something you shove in your pocket or waistband.
I think once you actually look at some of your options, you may come to the realization that the question here is how to best spend the money. Do you want to go full stealth - in which case you probably want to mothball the AKGs until you can afford a second open rig and get a micro kit from scratch, sorry can't help you on that one. Or do you want to forego major stealth capabilities and add a more "professional" all-in-one back-end to what you have. In which case I would definitely look hard at the Busman modded Fostex FR-2 as the best feature set of what has been mentioned so far.
Sorry, that might not be exactly what you wanted to hear, but like I say, go look at some of these boxes and decide for yourself what is really "stealth" and what is not...