First thing size, as everybody has different size ear canals but mostly M size fits all. You can mix & match with other IEM's silicone tips like i do from my collections and the fit/sound are tip dependants. The supplied stock foamies are not so good but OK as a free bie and does its job..but its really a serious challenge to me when it comes to replace as the foam will be little/bit more apart from its rubbererized shaft. Other than that its ok as we cant expect comply' quality.So remember to be cautious while removing the stock foams it might as well could possibly rip apart the swivel nozzle over period of time/usage.
Meelec's tips are also alternatives to the stock if you want to giv a shot. Try wider bore tips for more that you would expect from these type of IEMs.