If you are coming from IEM's, I suppose the soundstage of the PK2 might feel big. It is good for an earbud, but there are better.
My favorite out of the Yuin PK2, AKG K319 and ATH CM700 is the ATH. It has got good width and depth to the soundtage, but it is not very tall. I seldom think about how tall the soundstage is, but after comparing to other earbuds, the CM700 does feel limited in how tall the soundstage is. Like I said, I like it for it's depth and width together with good clarity.
The K319 has got a taller soundstage and about the same width. The depth and clarity however, seems to be slightly lower. There is a bit more lower end to the K319 than the CM700, which has got a leaner sound (probably why it appears to have more detail).
The PK2 is like the K319, but a bit more airy: pretty wide soundstage and taller than the CM700, but not as deep as the CM700.
If I could come with another suggestion, how about the Blox M2C? It is made by a small Chinese company and only made in small batches, so right now it is sold out. It has got a wide and tall soundstage and very good depth. However, I feel that it is not as detailed as for example the PK2. Still it slightly edges out the PK2 in my opinion. Blox is working on an improved version of the M2C, and apparently they are working on a high end earbud called BXIII. Don't know when that's out, though...
If you can find Creative Aurvana Air for a good price (around $100) it might be a good investment. The Head-fi member ClieOS, who knows a lot about different earbuds had this to say about it:
Aurvana Air reminds me of Audio Technica earbuds (the old CM series) - lean, neutral, detail, and airy with a really good soundstage. It doesn't have anywhere near to best of bass but it has really good speed and impact. It is the kind of earbuds that makes acoustic, instrument and classical sounds really good.
Aurvana Air was never quite a popular earbuds because Creative set the MSRP too high ($250~$300 IIRC). The current price drop ($99) is typical of the company trying to make a quick buck to clean out stock, which also makes the not-quite-popular earbuds becomes even rarer - that is unless you are in Singapore where the stock is always plenty as most of the local doesn't want to buy local.
I suppose you came across me at Anything but iPod? I sometimes hang out there as well. But there are more earbud discussions here. I've never been into IEM's, and with the earbud market being so small these days, there are not many places to discuss or read reviews on earbuds.
I have recently become enamoured with old Sony and Aiwa earbuds from the 80ies and 90ies. They can definitively measure up to modern earbuds. I have to say that the best soundstage with the most detail I have ever heard in an earbud is the Aiwa HP-D9 (made 1990-1995). It is very lean on bass, but oh, such depth in the soundstage and such amazing instrument separation! I treat my D9 like a full size headphone - I never take it out of the house, since it deserves to be listened to in a quiet environment. I was lucky to get my pair for 111 euro (they came with a broken Aiwa DAT-recorder that I have no use for, just wanted the earbuds). I have seen them go for as much as 300 euro! But they are quite rare nowadays, seeing that they have been discontinued for 16 years...
A comparison between the Aiwa HP-D9 and the old Sony earbud MDR-E262 (made 1984-1987) can be found here. It is amazing that even if technology has advanced in 25 years, the sound of earbuds really hasn't...