I replaced my E604:s with these and have not regretted a day. The frequency response seems wider and allows more flexible sound shaping by equalization, which has turned out useful when recording. The sound seems to have a bit more punch as well. I use these for toms and snare on stage and studio. Some engineers seem to prefer E604 for toms, and I understand that because they sound more like the traditional reference tom mic Sennheiser MD421. I however prefer the slightly more modern approach, neutral and open sound provided by E904. It's like a choice between apples and oranges, really.