You could try opening the box and running it with a large room fan blowing on it instead of the case fans. Some chance that will work, but YMMV. I was able to do that with a bunch of ASICMiner blades back in the day. It was still somewhat loud.
Don't do this! I can already tell by the way things work now that it would not be effective, you need fans blowing through the blades. The one nearest the power supply runs several degrees hotter because of less airflow, already.
I don't know what CMFs the fans currently in the case are moving so I don't know if these are an option but Silenx makes some pretty awesome fans, really quiet. They do a great job at getting the heat out of my PC, but I'm kinda doubtful that they would keep up with the heat in the S2, possibly. I know they have one that does around 90CFM for less than 20 dBA (I think 16 or 18). I am sure there are other options... find out how much air these fans move and then go from there.