What is this? Is it really just 'a bit' or is it significant? Does it increase extension as well? Does it have any drawbacks?
I've been somewhat inclined to give the ER4P + S adaptor a shot again after all these years... but the only thing holding me back is I have an HF2 and flat out dislike it. Is a modded ER4P a totally different animal from that?
Knowles makes a number of dampers, or filters, that adjust the sound of the upper-end of the drivers. They don't increase anything, ever, they are filters and dampers. However, the end result gives you a perceived increase in bass, although it's not an actual one. It's similar to the way Phonak uses a mid-range filter to adjust their sound. So what the filters do is adjust the treble. Some filters will end up "adding" treble because they filter out less than the ones that come installed on the Etymotics. Others will filter more to reduce the treble a bit more.
These filters do work on all Etymotic models BTW, not just the ER4. Etymotic sells green filters, I'm unsure if they are the same knowles filters, or developed in-house.