Mapman,
Did you give up on IEM at this point?
I have heard the B&W P5 headphones at the Apple store off of an iphone (mine with losses files). So take my comments with a grain of salt. They were built very nice but I found them pretty uneventful. They sounded pretty run of mill to me. They did not have any glaring flaws but they did not shin at all. I run my Ultimate ears off my iphone most of the time and they sounded MUCH better. Lower bass (cleaner too) more detailed and better impact from top to bottom. For $300 there are better headphones than the P5.
When looking for headphones that are easy to driver look at how many ohms they are, and the sensitivity. The lower the ohms the easier they will be to drive but the louder the noise floor (hissss) will be from the source too.
If you are not against on the ear headphones and need something easy to driver, Grados will fit the bill. They are sort of like horns to me though, a little colored, a little bright, but have great dynamics and impact to off set the flaws. The Grado RS-7 is the one headphone I wish I did not sell.
A brand called Alessandro (link below) takes Grado headphones and modifies them to be a little more flat. They keep all the good points of Grado but make them a little more tonally balance. I owned the Music Series Two before I bought the Grado RS-1. But the Alessandros also fall on the bright side of neutral but are a lot of fun. Built quality like all Grados is pretty iffy though.
Alessandro High-End Products Headroom (www.headphone.com) is an assume site with reliable reviews of the headphones they carry. You can also overlay frequency graphs of different headphone phones they cary on their site. They measure them in-house.Their own line of amps holds their own too.
Also Head-fi.org is a very helpful and knowledgeable headphone forum.