You are probably right about the bigger fish being deeper.. If that's where you're seeing them, then that's where they are. Many folks recommend a strike indicator when fishing sub-surface. You can adjust the strike indicator to what ever depth you want depending on the depth of the target fish. In moving water, especially fast moving water, good line mending technique is paramount to success. Mending, in short, means throwing the line above your indicator upstream of your indicator so that the current in the water does not cause your line to drag your fly through the water faster than if your fly was just a free floating bug. As far as fishing deep in stillwater goes, Brian Chan has probably the most successful fishing technique I've seen. Google his name and you should get lots of help. I hope my .02 cents helps.