있으면 = if there is (direct, firm assumption). 있다면 = if we suppose/assume there is (softer supposition). It's because 다 in 있다면 comes from the quote form -다고 (말)하다. So 있다면 is like a short form of 있다고 한다면 (if we say there is ...). 다면 is usually weaker than 으면, meaning there is more uncertainty.