I went through this quest about a year ago. I contacted A Pure Sound at least 3 times to place an order and never heard anything. Perhaps he is no longer in business? Anyway, since that route didn't lead anywhere, and since my usual source for cables (Drew at Moon Audio) doesn't offer anything that is compatible with Etymotic 'phones, I ordered the Lune Series III replacement cable from Null Audio, in all black. It took a while to arrive, but when it did, I was very pleased.
First off, the cable is soft and very flexible. It almost feels like a shoelace. There is a harder piece around the ear (feels like shrink wrap) to hold it in place behind the ear. The Etymotic-specific connector is rather large compared to the plastic piece on the stock cable, but the connection is very solid. On the other end, I went with the standard Null Audio SC3 3.5mm (1/8") connector. It is also very solid and the gray finish looks nice next to the black nylon sheath. Overall, they are really nice cables to use - they are comfortable, don't tangle, and stay in place behind my ear. There are no microphonics to speak of.
How about the sound? In short, they sound fantastic! The best way to describe the subjective change in the sound through these cables compared to the stock cable is that they sound more organic. By that I mean the ER-4s are very detailed and revealing, and they lost none of that, but it feels as though the music breathes a bit better through these cables. There is a greater sense of ease. Perhaps something about the impedance of the cable or the connector tames a specific frequency spike to which my ears are particularly sensitive - who knows? It's like the sound just relaxed and settled in for the ride rather than being on alert.
The other thing I noticed is a coherence to the sound top to bottom. With the ER-4s, it's definitely not "all about that bass" and I don't mean to imply that the cables changed the extension in either direction, exactly - but with these in between my amp and my ears, I found just listened to the music rather than listening to the frequency characteristics of the 'phones. The difference is subtle but important - these cables bring out the music lover rather than the audiophile part of my brain.
For context, I mainly listen to classical (especially well-recorded chamber music) and jazz. When I travel, and sometimes at the office, I listen to ALAC files ripped from CDs and stored on a 160 gb iPod Classic played through a Moon Audio Silver Dragon cable into a Fostex HP-P1 dac/amp. I have the ER-4P and the Etymotic-brand ER-4S converter cable, which I tend to use most of the time. (The HP-P1 has a volume imbalance between the L and R channels at the lower end of its volume pot, and it happens to be right where I like to listen without the converter cable in the mix. By inserting the additional resistor, I can up the volume and get past that.) I find the entire presentation of the ER-4 with the S converter to be more relaxed and organic (essentially what I was describing above) and generally prefer it. (In short, I think the Null Audio cables bring out the best characteristics of the ER-4S variant without adding any unwanted peakiness or coloration.)
My other set-up is a 13" MacBook Pro (retina) with high-res files (aiff) downloaded from HD Tracks and played through the Amarra 3.0.3 player. I have a Dragonfly 1.2 as well as a Meridian Explorer2 (fed by a Moon Audio Black Dragon UBS cable) and generally prefer the Explorer. High-res recordings through the Etys are incredibly detailed and engaging, and I found that the more organic, relaxed character of the Null Audio cables enhances that sense of spaciousness around the instruments.
I recently got a great deal on an RAL KEB02iB, which works with both the iPod Classic and the MacBook Pro. In that configuration, I have to use the S converter because the RAL has a ton of hiss throug the Etys without it. With the converter in place, it just seems kind of lifeless. It is not a synergistic combo. (However, the RAL is absolutely incredible through open-back headphones, like the Beyerdynamic DT-880, 250 ohm version! Talk about organic and musical! It is by far and away the best music listening experience I have had outside of a concert hall. Too bad it doesn't work with efficient iems.)
Null Audio has other cables that are more expensive than the Lune series cable I ordered. I suppose it is possible that they might sound different - even better - than the Lune version. Let's just say I am usually the kind who suffers from upgrade-itis and I don't feel the need to upgrade these. I am very happy with them and would definitely buy them again.
