Fandangoya, you're in a similar situation that I was in; I upgraded from the eH-150 to the Sennheiser HD-555.
Why do you dislike the regular adapters? If you hate having a protrusion from your headphone jack, why not buy a short headphone extension cable, such as those made for the iPhone? It sure beats a horribly overpriced adapter from HeadRoom.
DealExtreme: $1.60 3.5mm Stereo Headset Converter Cable for Apple iPhone (14cm)
Now, I'll tell you what I thought of the upgrade, in short. The soundstage is by far the best thing about HD-555; it truly immerses you in the sound, far more than the eH-150. The sound is also warmer, giving an at-home feeling (if that means anything to you). I don't consider my ears to be trained enough to talk much about midrange and treble, but it's there and it's luscious.
There were two things that didn't excite me so much. First was the detail; The HD-555 is very slow, and it smoothes out the sound. This is great for sounds like strings, but not as good for sounds requiring detail. Also not so exciting was the lack of bass. The eH-150 had wonderful bass, which to my inexperienced ears was precise and. impactful. The HD-555 certainly didn't have bloated bass, but it just wasn't present enough. And I'm the kind of guy who detests subwoofers and doesn't care much for heavy bass.
However, all was not ill, for I discovered burn-in! I had long been skeptical of what I thought was a placebo effect until it happened. Something magical occurred, and the two quirks that I had with the headphone disappeared! Detail flourished, while somehow retaining smoothness and silkiness of the sound, and the bass started popping up. Not as much as on the eH-150, but just the right amount for me. Everything is absolutely lovely now, and I fully recommend the Sennheiser HD-555 as a great upgrade to the eH-150.
With regard to amplification, there's no real need. I run my headphones out of the 'mid-fi' amplifier in my receiver most of the time, and they sound great. Of course, most people don't have a home theatre system everywhere they want to listen to headphones. Thankfully, you'll get all the volume you need from the itty-bittiest iPod nano or other MP3 player of choice.