[granolini]

I'm in a similar boat with my app and we've found success by implementing the MomTest (http://momtestbook.com/). The premise is that your users are well-intentioned and will tell you what you want to hear, just like your mom would. To work around this, you employ the MomTest- basically you avoid all questions with words like "would you", "do you", etc because they lead to red herrings and reassuring dead ends. You ask questions about their real experiences relating to the problem you're trying to solve so that you can deduce the rest. Your mileage may vary but since you have some existing users, you might consider doing this to extract some useful data about them.