I am big believer of ear tips in general. They have ability to change sound signature. But in general foam tips provides the most amount of bass. Silicon tips can also vary depending on the material used from soft to hard etc. Then there is size variation.Help me understand how an ear tip can change the sound signature of an Etymotic? Do they somewhat plug up the hole in the end? Maybe obstruct a portion of your ear canal? Since discovering Etymotic, that's all I use for portable listening. I've experimented with all types of ear tips and none of them actually changed the sound. The only thing different was the level of isolation. Are you certain that's not what you're experiencing?
It’s similar to room acoustics in which placement of the speakers can change the sound perceived.