^well..that would confuse you haha. By punch, I mean the impact and sound you will feel/hear is in quicker, shorter bursts which result in a more exciting sound. The Pro 900s have a louder, bigger bass which does hit harder, but it is not as quick. It sound more fluid and warmer.
Stock for stock, I still like the 780s more.
Technically, the 900s are a better headphone due to their improved detail and bigger sound stage. However, they lack the excitement of the 780s and I felt the 780s weren't far behind the 900s in detail at all. The soundstage is of the 900s is noticeably bigger, but again, this wasn't a deal breaker for me. After listening to both (stock 780 and Kees' mod 900) for about 10 mins each, it was easy choosing which one to keep.