The diff between 3.5mV and 5.5mV is less than 2 db.
Lew, make that "less than 4 db." As I'm sure you realize, it would be calculated as 20 x log(5.5/3.5) or 20 x log(3.5/5.5).
Which of course is still consistent with the rest of the statements in your post.
Best regards,
-- Al