If you want to know what it's like to deal with your business and how to improve, why would you not consider going to the source?
The key is to ask the right questions. Try and find out what clients don't like, rather than simply what they do. Client surveys can only give you so much insight. To get the really good stuff, it needs to be a two-‐way conversation where you can explore feedback in more detail. Over lunch isn't a bad option, but make sure you're talking to the type of client you want more of.