The "flooring" you're seeing might be due to a problem in the AM signal applied to the summing amps after the Johnson counter. Since the output of the Johnson counter produces unipolar signals the AM circuit has to provide an offset to make it bipolar. If the offset doesn't get the signals to be symetrically bipolar you'll hit the rail when AM is turned up all of the way. The other possibility is the summing amp has too much gain, but in your image it looks like you're only hitting one rail...