With the amount of rain we've been getting, it's very likely that too much water is at least part of the problem. One source I've found says that Iron chlorosis is a symptom of overwatering. My understanding is that Saaz tends to prefer wetter climates, which might explain why it's doing better than the others. However, some symptoms like the curling/black leaf tips were showing up before the heavy rains, and the Sterlings have never looked all that great to begin with.
Anyway, we're finally in the midst of a decent dry spell, so if the problems do stem from too much water, hopefully they'll start showing some improvement pretty soon. Otherwise, I think I'm getting possible nutrient issues narrowed down.