A few points in no particular order...
The W5000 will have poor isolation regardless of how tight the headband is, because there's two slits hidden on the sides just behind the pads that lead almost directly into the ear chamber.
If the W5000 headband is too loose, the sound will be thinner than it should. You can bend the headband to make them clamp very tightly, but the bass quality of them will still be only a fraction of what it is on a modded pair with the headband still loose.
I don't think the W5000 is specifically either source or amp dependant per se, but rather very critical of any upstream components. Not so much in the typical audiophile "these are so good they need only the best", but more in the fact that their extreme sensitivity will highlight any noise, hiss, interference, distortion, etc on any upstream component.
I agree with the others that the W5000 is more detailed/analytical/brighter and the W11JPN more fun/lush/euphonic, but there are mods that can make the W5000 more euphonic like the W11JPN/L3000 while still keeping most of it's own unique sonic character in the midrange.
The headband on my own pair always looked fairly big to me, from the pictures I've seen of other pairs, but even when I bend it into a looser fit it still stays on my head with no problem.
-Fitz