Culture is an interesting thing. It begins to emerge as soon as a second person is involved in your business, yet so many founders wait until it's beginning to become a problem before they deal with it.
I've worked with lots of companies (large and small) to create engaging cultures. The biggest issue is often found in your question. You want to keep your company "the cool place to work at" ... for whom?
Leaders often think they can define and control the culture. The best they can do is influence. The culture of a growing company must leverage the best of everyone who is part of it - not seek to turn every new employee into a copy of the founder.