I've been through pleny of earbuds in the lower price range for use while running trails or in the gym. I eventually (with the help of this forum) found the M6's and really liked them, except the supplied tips always produced an "odd" seal in my ears. By "odd" I mean that they can sound awesome when first inserted, then when I move my head a certain way, they seem to re-orient their shape slightly and I get this weird "out of phase" sound. The seal is still there so there's plenty of low-end response, but it seems that the tip may be taking on a slightly different shape in the ear canal affecting how the tip delivers the sound into the ear. When I return my head back to position, the out-of-phase effect typically persists until I remove them and re-insert.
So, I bought the Sony Hybrids based on suggestions here. I get the same effect.
I use the large tips (both Stock and Sony). Figuring that the larger size is just de-forming in my ear, I tried the medium, but they just don't create the seal needed for isolation and low-end response.
I know I can buy Complys, which I use on my Shure 215's - I love 'em. But I am not sure I want to spend $17 on tips for $19 earbuds. Maybe I'll have to?