I know that the difference between the E3 and the E3c is that the E3c is white. What is the difference between the E5 and the E5c? For one thing, I noticed at MicrophoneSolutions.com that the level attenuator appears to be optional while at the Shure website it appears to be included. [this could be an incorrect interpretation on my part...] Does anyone know for sure if there are any differences between the two?

Yes, E5c comes with level attenuator, which is an extension cord with a volume switch on it, it degrads the sound quality a bit, and not necessary in most situations. Nice to have for those times you have a source that's too loud and doesn't have its own volume switch.

E5c also comes with a nicer carrying case.. it's a round case made from microfibre.. nice little nifty thing.

Otherwise, I think they come with all the same sleeve fitting now (although there might be old stocks of E5 out there without the new soft flex sleeves).

There are no other physical or spec differences between the E5 and E5c.