The DT 880s are lovely headphones and also very comfy around ears
That SS valve amp looks awesome - let us know how much of a difference there is to the sound quality - not just that it goes louder with and without
It's not really much louder, just a much fuller sound, more detailed etc. Apparently because they're high impedance they need quite a high voltage to drive them and the valve amp does that better than transistors. It's meant to be common practice to drive them from the loudspeaker terminals of a regular power amp, but I've never been brave enough to try that.
Sorry to hijack this thread.
You can return items to Amazon, but you'll probably end up paying shipping (which is only fair really). In reality all the headphones you've mentioned are very good and I honestly don't think that you'll have any problems with any of them. A lot of what you read online is really, really an@l stuff about the most insignificant minutia of details (try the HeadFi forum if you really want to expose yourself to it, but don't blame me when you run away screaming....).
In summary, Beyer Dynamics are probably the most neutral of the bunch and the DT880's are more open than the 770's or 990's in terms of soundstage etc. The Senn's are generally thought of as being slightly more 'polite', but the 555's, 598's are still superb headphones'. Grados tend to be more forward (i.e. a little brash, but great for rock and guitars) and some people find them uncomfortable and bit geeky looking. I don't know AKG well enough to comment, but have always fancied a pair of 701's.
None of the above is meant as criticism, it's all just down to personal taste.You won't be disappointed with any of them. Buy a lower impedance ie. 80 ohms or less and they should be okay on the DP. The higher ones are meant to be slightly more dynamic as the voice coils are a bit lighter (but then again....).
Buy a closed back version if there's people around who'll be disturbed by listening to what you're playing !
If you do want a recommendation, then I strongly recommend the DT880's. I'm sure everybody else who's posted will say the same about their own headphones, which again, isn't criticism, it's just that they are all really good.