Those two headphones really aren't that far apart in terms of "quality", and even pricewise they're not too far (a Beyer DT880pro is around $220, a HD380 around $180; premium models mostly just add flair)... and Sennheiser is just as "high end" as Beyer and AT. I really don't know what you're expecting here. The audio game is particularly bad for diminishing returns.
I'm personally not a fan of the Beyers and would pick the HD380/PXC450 over them. That said, I do prefer the Beyers out of a tube amps and think they sound terrible out of solid state (including Beyer's own A1 amp). I've heard $500 headphones, I've heard $1k headphones, I've modded my own shells, transplanted orthos, and built my own amps. Out of all that, one of my favourites is still the little Senn PX100-ii out of my Fuze or from my own cmoy style amps. There's just no correlation between budget and taste sometimes.
I see. Well, I already have a soundcard (not an expensive one like the Auzentech X-Fi Forte) so what's your opinion on adding an external DAC (and recommendation would be awesome as well in the case it's worth it)?
I was lucky enough to find my headphones at a store today where it was hooked up to a CD player and a headphone amp way more expensive than my E9 (Didn't write the name down...), and I'm positive they sounded way better than what I'm getting out of my PC, so I suspect there's a bottleneck somewhere? Or am I totally lost here?