I have these at my place for a review right now with an aftermarket cable (5n Silver plated OCC). They're at about 100 hours of burn-in and they're still very sibilant to me.

Very sibilant? Not my finding at all. I used the stock cable, not that I think there should be any big difference there. Some people on HF said there was a big difference with different tips, etc., but I think that might be more of a statement about fit. Certainly, get a totally full seal, no vacuum and seated well, but not overly forced in or pulled out.

The RD3 has a treble bump to it for sure, north of 5k. I think someone would have to be pretty sensitive to treble to hear it as sibilant, based on what I heard (which is possible, people have hearing damage all the time). I have a feeling you can bring it into tolerable range with a lot of playing around with fit and tips, even if it's still not a good tuning for your ears. Remember, angle of insertion is as important as insertion depth. Good luck, badluck.