IMO it is very hard to breed traits into dogs, if it were that easy we would all breed champs! Looking back through your bitches lines , she did have some strength with Nurse Perkins being a staying bitch so it is in there somewhere! Personally I would look very closely at the formguides and see which Sires are throwing the 600-700m dogs more often than not and give them a try ( with no guarantee) or try one from left field like Texas Gold, Miagi or Chinatown Lad. Premier Fantasy is an outcross and seems to throw strong ones!! At the end of the day you have got a good producing bitch who seems to throw to anyone so you will get winners anyway, good luck hope you get some city winning pups next!! All this waffle is just MY opinion!!!!