And I'm back (it's been a few days since Thursday, busy weekends blurgh) with some nice findings. I hope this can help some other people in search for a new pair of headphones.
So first off, the general advice to try the headphones before buying is golden. There are just so many variables that can be a deciding factor in whether you'll love the cans or hate them.
The store I went to had most of the headphones I was looking at in stock, and were kind enough to let me test them straight from the box, and with an amp they had for sale as well.
The ones I've tested were:
Beyer DT770 Pro (250 ohm)
Beyer DT880 Pro (250 ohm)
Beyer DT990 Pro (250 ohm)
Beyer Custom One Pro (16 ohm)
Sennheiser HD598 (50? ohm)
I've tested them all with direct output from iTouch 4th generation and with an amp. It costs around €150,00 but I can't for the life of me remember what it's called again.
My (personal) conclusions/opinions are this:
Sound quality
Note: the headphones all came straight from the box and weren't burnt in. Sound quality might change a bit after this occurs.
The Custom One and the HD598 worked great unamped. They were much clearer than the Beyer DT's, whose sound was low and muddier. The iTouch was able to drive them (in my eyes/ears) properly, where as the DT's just need more oomph.
When using the amplifier though, the tables turned. The €50,00 more expensive HD598 could not get to the same level of sound the DT770 and DT990 could produce. The DT880 was a step above that, again, but at additional costs (€225 for the 880's and €145 for the 770 and 990's). The 598 did benefit a little from the amp, but not nearly as much as the higher impedance cans did.
The only 'issue' I had with the 880's was that it's just too honest, or confronting if you will. It's very neutral compared to the other cans. I'm sure there's lots of people who love this when listening to music, but it's not my cuppa' tea.
When it came to the 598 I was a little afraid to be disappointed, because reviews on it say that it is lacking in the bass department. While there was less bass than the Beyers it wasn't shockingly so. The bass was quite pleasant to my ears
because it wasn't so prominent. So again, reviews are fine to get a general feel.. but you're not buying cans because of how they feel now do you
.
For me the showdown was now between the 770 and 990's. Even though the 770 is (I guess) supposed to be the more bassier can due to its closed nature, the 990 had more of it. More punchy, deeper and more prominent. What the 990 also had was quite noticeably recessed mids, which made some of the songs sound a bit weak and sad, while they are upbeat songs. Soundstage on the 990 was bigger as well. The sound really came from all around you, while not seeming to come from headphones.
The only odd one out is the Custom One Pro. It sounded quite like the 770, but was more expensive and less comfortable.. and speaking of comfort;
Comfort/build quality
For me the Beyer DT's take first place here as well. The Sennheisers felt cold and plastic-ey and clamped quite hard. The Custom One Pro felt quite cheap and brittle, despite it being more expensive than the 770 and 990's. There wasn't a whole lot of difference between the DT's. They were all about equally as comfortable.
Purchase and happiness
In the end I went home with the 770's. While the 990's were a close call, I couldn't get over the clarity the 770s had. They also did not break my piggy bank, as I was somewhat dreading that as well when looking at the 880's.
Initial testing at home with the Xonar Essence STX (which has a built-in amp for headphones) is remarkable. Perhaps an equally expensive stand-alone amp would give me better quality still, but I do like to fiddle with Dolby Headphone and what not for movies.
I'm also rediscovering my entire music collection as we speak, and I'm loving it. Positional audio is not even in the same league to be compared to my G35, which has now been demoted to microphone when I use Skype.. I guess I'll need to find a worthy desktop microphone now.
In total I've spent around €300,00 and I do not regret it a single bit. My parents were calling me all kinds of crazy when I told them, but that soon changed when they had a side-by-side comparison with their iBuds and garbage €10,00 on-ear headphones.
Thanks for the help and opinions that have been popping up in this thread, and in other topics as well. This is a great community from what I've seen.