The way they describe them on the Noble website I find to be very accurate. The parts below in italics are from the Noble pages.
3's = With a ‘V’ shaped frequency response, the Noble 3 in-ear monitor is characterized by a controlled, impactful, low end and far-reaching highs.
I like the 3's and they're a great entry point to IEMs. They're definitely V shaped and fun to listen to, but not in the same class as the bigger siblings (6's and 10's) in my opinion in terms of SQ of my liking. However, they're very good for their price-point.
4's = neutral sounding piece designed to appeal to those who desire a flatter frequency response... smooth, linear, sound makes for an ideal pairing with vocal, acoustic, and classical music.
The 4's are very neutral, much more of a "reference" style, if that's your cup of tea. It's not mine, I like engaging and fun, but I know many people who love these.
5's = deep, thick layered, bass and highs that never fatigue the ear, the Noble 5 in-ear monitor strikes a balance between fun and articulate.
Can't say much as I have very limited experience with the 5's.
6's = the low-end is considerably filled out while soundstage has expanded building upon what is already a natural, airy, space...monitor is characterized by smooth mids and highs with great extension at the top end.
The 6's are really fun and if I couldn't afford the K10's I'd be happy with these. I'd say the 6's are more like the HD650's, which are my backup cans to my LCD-3's.
K10's = seemingly unchallenged at every frequency and capacity. 10-drivers working in unison as one results in what is likely the most coherent sound ever produced by a portable audio product.
...but luckily I can afford the Kaisers and have owned mine for well over a year now. Love them to death, and you can find the link to my full review in my signature line. You'll also see how I compare the K10's and the LCD-3's in that review. That's the best comparison I can make.
Cheers