[KimChee]

Definately need an amp for the 4P, it brings it alive!

 

[mertz]

The amp seems to help the freq response to even out. ie, adds bass to an otherwise vacant curve.

 

[starlethe]

There is always a p to s prepared for 4p, which makes it really convenient to change the sound. However, there is still a little difference between 4s and 4p+p2s, so I tested and found out that p2s features 68.3 ohms which is slightly smaller than expected, maybe that is the reason count for the difference.

 

[drMuse]

I bought an Etymotic Research ER4S set about 20 years ago and ordered a headphone amp for it, "Airhead." Did that because threads I read included info that a headphone amp was required with them. My personal experience, however, was that I was getting sufficient audio levels with my portable devices without using that amp. Therefore, I never used that Airhead amp with the Etymotics, except to test/experiment! I do use that Airhead amp now, but only because I want access to volume control at my listening location from a device that doesn't support doing it remotely (my 301 disk Pioneer CD player stack... two of them). Those ER4S lasted me over 10 years without a problem. Eventually I had cord discontinuity issues (I figure, don't really remember, but had to be that) and I have 3 sets of ER4P now. For whatever reason I have much greater issues with cord discontinuity now than I ever did with those ER4S. I figure maybe the cord systems just aren't nearly as robust as they used to be.