German blacks are supposed to be fierce robbers. From some stuff I read that is one way they used to wipe out competitors in the wild (Italians, carnies, etc) and is why there were always a large feral population of them till mites cleaned house. Im not saying thats what they are but they could be.
Ok, Im going to off topic kinda but I have had a bit of a crush on a.m.m. since I started reading about bees and I have always wanted to catch some feral ones. It was until I heard more about them recently and understood why they were disliked so much that I decided not to get some until I have a yard where they can not mate with the rest of my stock. You say they are beating up your bees reminded me of this below...
Here is a quote from Robert Russell about A.M.M. who is as far as I know the only breeder of A.M.M. in the states. Robert Russell is a very large and respected queen breeder. You can check out his website and forum here.http://russellapiaries.webs.com/
"AMM are great bees indeed... They are extremely strong and can over whelm other genetics in an apiary by simple natural selection... They are heavy robbers and produce more drones naturally than any other breed... That is why they have maintained their standing in the US even as millions of hives produce swarms of different strains each year... the AMMs (and hybrids with AMM in them) will rob everything else and produce enough drones that even what they do not kill will eventually start taking on their genetics.
I will say however, there are some concerns that one should have when keeping them... they are hard to keep near other colonies when there are no flows... during fall, they can rob weaker hives, but during winter, each warm day will pose a new threat... as your other colonies cluster and their numbers dwindle, the AMMs can take advantage of them as well.
With this in mind, keeping a good eye on your flows can prevent losses... placing entrance reducers during dirths will help greatly.... Also mending equipment that has holes in the corners... Feeding can be risky too... and has to be done only during build up or right before each flow so that they dont start to rely on it, and thus start robbing when it runs out.
That being said... They are certainly the most adaptive bees that I have ever worked with."