I’ve recently had a Fiio E6 headphone amp delivered with an L9 cable for the iphone 4s.
So far i’ve tried it with 6 pairs of headphones ranging from the basic apple ear buds to a couple of +£200 in ears and +£500 over ears.
All sound dramatically fuller with nicer separation and depth.
Three of my colleagues who are “into” music are also equally amazed.
Stunning bit of kit for £25
Obviously wont replace a proper amp but for a potable and cheap bit of tech, it’s an awesome buy.