I've tried setting a static diff for the S9 as suggested (password: c=BTC,d=262000) but the pool doesn't use the diff setting, and the diff changes dynamically.
Recently we altered the sha256 stratum difficulty to allow some people who don't necessarily own Antminer S9's the opportunity to mine using their equipment as well. Essentially anything over 250k static diff would trigger the variable difficulty as a result of this. We just haven't edited the text yet on the main page. sha256 has one of the widest range of possible connection details and we have been tuning it to match who shows up to use that algo. Nicehash currently requires 1,000,000 difficulty with their plan, some miners who own S9 prefer 262k on strictly Bitcoin mining pools, but here var diff works well because a lot of the options presently available are not that difficult to find and process yet. As the number of miners expand we have at least 10 more coins ready to enable which we'll tune the difficulty even more for better pool results.
We are still working on enabling a higher difficulty port for this situation as well, but we want to make sure everything is recorded properly between the two different ports before we enable it globally. We'll post here and on the site when we have more to announce concerning that.