You know so far - I've just stuck with the tips that were on them (medium size single flange). They seal perfectly every time I stick them in (or so it seems to me - maybe just because it's so perfect in comparison to all the other IE's I've used before).
The other really cool part about "the seal" is that it takes a direct pull/yank on the cord to loose seal. I can move around all over the place and the seal won't break at all - which is the opposite of every other IE I've treid.
Sound Isolation: With no music on at all I can turn the shower on and maybe hear 20% of the water noise if I'm standing in the open door. Or, I can turn my vent-a-hode on high and standing right next to it maybe only hear 20% of the noise it's making. 20% may be off a bit, it's nothing I've really thought about yet. My own voice is really loud even when music is playing loud though - a lot more than I've ever heard before. I *think* that's because the sound isolation (seal) is so much better than anything else I've experienced though. :p
I'm one very happy camper. :)