Hi kova,
As I understand it, there are a few factors that seem to influence the way a particular tips sound - besides whether or not you get a good seal with them. One is the width of the aperture - generally given the same tip material, the wider the aperture, the more treble there is in the sound. This has something to do with acoustic impedance, and I believe that as the tip becomes more narrow, the more difficult it is for sound waves to travel through the passage of the tip, and the more bass there is in the sound. Perhaps someone else can actually tell us why that is the case. Another thing to consider is that the material of the tip is important, in that a softer material may become more squashed easily, which again effects the amount of acoustic impedance.
The other factor is the actual material of the tips - foam, silicone, hybrids, etc. I'm not sure why different tip materials affect the sound beyond changing the amount of sealing, but generally I've found foam tips tend to warm up the sound.
The last thing I can think of is the shape of the tips. A tip which sits flush with the nozzle generally sounds more detailed/intimate than a tip which extends in a way that keeps the IEM further away from your ear - conversely those longer tips seem to increase soundstage.
All these factors are influenced by how the bends and changes in the ear canal actually effect the sound. Full customs are able to bypass a lot of the influence of the ear canal shape by plugging quite deep into the canal.
Sorry I couldn't be more specific, but I've generally found, playing around with different tips with different IEMs, that there is generally no magic formula for finding the right tips. Some tips work fantastic for some phones, even when you wouldn't think it, and some are terrible. It honestly surprises me each time how much of an effect tips have on the sound - sometimes in bizarre, unanticipated ways.
