Sennheiser CX 300 series was formerly launch into the professional musician market as Sennheiser IE4 In-Ear Monitor.
Sennheiser IE4 sold at ~150 US Dollar (Suggested Retail Price). As a competitor of Etymotic Er6 and Shure E3c.
Following the product's success in the professional market now it is time to harvest from the vast consumer market.
So the marketing description of this product now change from "the high accuracy reproducing of the sound" into the new hype "the bass driven sound".
Very common marketing trick.
Sennheiser IE4 was a rebranding of the highly acclaim "Sharp MD33".
Sharp MD33 is one of the most succesfull IEM in the world.
It also described as a "giant killer".
After it got wide reputation around the world Sharp MD 33 was rebranded into Creative ep630 (which can be found in Asia) and Sennheiser IE4 (which goes directly to the most demanding professional musicians market.
Sound of this Sennheiser/ Sharp/ Creative IEM is solid and very clear.
Bass is decent, mid and high are details.
It need less maintenance comparing with Etymotic or Shure since it cleverly use metal grills instead of direct filter.
Once a month scrap your earwax out of the metal grill is enough to keep this decent IEM working for years.
Silicone ear tips are interchangeable with Sony's ex series ear tips.
If you want to do some experiment you can try Shure's ear tip on them.
Even somebody have suggested Shure's Ultra Soft Flex Sleeve (PA749M) on this IEM but I would like to suggest Shure's PA757M Flex Sleeves instead. (Please change the size according to your ear size)
With Shure PA757M Flex Sleeve this Sharp/ Creative/ Sennheiser IEM sounds as if the sound stage become expanded without deterioration in sound quality.
If you live in US this Sennheiser IEM is worth your money. And sound quality is much better than many expensive IEM.