PK2 earphones are the low impedance version of PK1. Although not as good as PK1, PK2's sound signature is similar to PK1. You don't have to drive PK2 with an expensive head-amp. If you are looking for something with high performance/price ratio, PK2 might be the best choice.
“The following is taken out of its context from my review here http://www.head-fi.org/t/687962/asg-2-ck100pro-and-ue900-reviews-the-dysfunctional-family I just thought that the CK100Pro deserved a review here. I don't own it but have auditioned it extensively over about 8 days. Though I just might buy it later... CK100Pro: The Sheldon Cooper of iems Contrary to the warm...” --MoonYeol
“ These were my sweetest sounding head-gear yet. It would be brilliant all-rounder if it would not require a portable amp for portable use. This is the first reason i left it, otherwise i almost got an ear infection from those foams.., they get dirty in a few weeks of massive use. I recommend them to clean, i boiled them in water. ” --angevoi
“ Well firstly this is not a true iem. The sound is about as great as one could get from an earbud. In my limited experience and without trying the pk1, I would say these are the finest buds you can buy. With that said, these are not for everyone, this is a niche product within an already niche market. I find driving it with my D12 anaconda makes for a pleasurable sound,...” --Jamesdeandoa
Technical Details * Impedance:32 ohm(at 1KHz) * Frequency response: 20-20,000Hz * Sensitivity:120 dB (at 1KHz) * Max Input Power: 400 mv
“ The PK3 is a great, warm earbud. It does vocals especially well because it is very mid-focused. It has impressive bass extension, not especially strong impact; the trebles are smooth and may roll off too soon for clarity and detail nuts. But the mids are what the PK3 are all about. ” --kmhaynes
If you want earbuds that will give you crystal clear audio at a great low price, the Audio Technica ATH-CK303M Earbuds is your choice. With a 20-20,000 Hz frequency response, gold plated connector, and a comfortable fit that provides maximum sound isolation, the ATH-CK303M Earbuds are an incredible value.