I got these guys: http://www.amazon.com/Etymotic-Research-ER38-18-3-Flange-Replacement/dp/B006WT2AD8/ref=sr_1_1?ie=UTF8&qid=1379682319&sr=8-1&keywords=Etymotic+Tri-flange   They fit perfectly and now my IEM's double as hearing protection! (almost but not really)

I just received my pair of R-50Ms and I haven't noticed and sibilance. But I will listen for it now. I did however change the tips from the stock to the often recommended Etymotic Tri-flange tips much better isolation and sound out of them.   Though talking on the phone with that much...

  I just got my R-50m's yesterday. Listened to them a bit and they sound great! Coming from the stock iem's that came with my phone I didn't think iems could sound so good.   I ordered them off the site you posted. It took 3 weeks shipping to Canada from China.