I don't know your gear specifically, but generally speaking, either a PA2V2 or Cmoy will enhance your listening.
I have not yet heard the E4c. I do have the E2's which may or may not have a Shure signature sound to them. With a PA2V2, I find that the rolled off high end seems somewhat recovered. In other words, through amplification, I hear some improvements to the high end and get better tonal balance in general without muddiness being added to the bass or the midrange clarity being affected.
Subjective audio qualities aside, the PA2V2 is a closed end black box. I find that it does improve somewhat from burn-in but it will stablize and sound the sound throughout its usable lifetime. As the PA2 is generally a good amp for the price, that's not a problem and when you factor in its ability to use low cost NiMH AA's and its ability to recharge them in the amp itself, there's good value there.
The cmoys are good amps as well and have spawned many derivative as well by numerous vendors. With the generic cmoy available, it is hard to say what the quality will be as cmoy is not so much a product as a design specification and you need to have some sense for a builder's reputation to know how well the amp may be built. Cmoys generally leave the opamp(s) socketed, thus allowed and intending for that critical IC to be user interchangeable. Apart from how well the circuitry is implemented every cmoy will sound a bit different dependent on the opamp inside. Given the plethora of compatible opamps available, the likelihood of finding an opamp well suited to the E4s is very high, so you have that flexibility there. At the $100 level there are some commercial cmoy derivatives to consider. One that I have is the Headsave Go-Vibe which I can also recommend. Another I know of but have no experience with is the Super Mini-moy. You can search the threads for this as well as the PA2V2, cmoy, and Go-Vibe and you will have enough information to make your head spin.
In general, all will provide audio quality beyond what the amp section of most portable devices can provide.