Hi Magnus I think I can do a pretty good job for you comparing these headphones as I upgraded the HifiMan Re0's to the DT1350's the other week...
The listening equipment was my Apple Mac using Fidelia Audio player connected to a FiiO E7 external USB DAC, Fidelia had exclusive access to the DAC and I used the same volume on both sets of headphones as they are very similar in their impedance / sensitivity.
The first track I used as a reference was the Massive Attack track Dissolved Girl from the Mezzanine album, and I am typing my observations as I listen to each headphone:-
Beyerdynamic DT1350 - Good bass extension, controlled not Earth shaking though, but it is there none the less (the bass is very dependant on the position of the headphones), it's more like you hear the bass as opposed to FEELING it. Midrange is there not too bright in the upper mids either but it is slightly recessed due to the closed back design as for the treble it is good, nice and clean. As for the stereo seperation it's very good as is the soundstage (hard to tell though with this recording as it is very narrow due to it's nature), oh and there is oodles of detail that wasn't there prior.
HifiMan Re0 with Comply Eartips - Smaller bass immediately apparent, seems to be be more low level detail though? Eww the vocals sound all nasally not nice, but the kick drum has more mid detail, the Re0 seems to be picking up more mid midrange detail (or lower treble, hard to tell) up than the DT1350. The soundstage is not as wide and the separation is similar to the DT1350. Ouch the mid guitar section is really quite painful, some nasty midrange out of control stuff there. Mmm hearing some things now in the track that the DT1350 missed (reversed vocal effect), but again where is the bass???
Conclusion - The DT1350 blows the Re0 out of the water bass and midrange wise, it's there and controlled where as the Re0 is a bit of mess, as for detail the Re0 has the slight edge seemingly revealing things the more expensive DT1350 doesn't show as well, also the Re0's sound tires me out, it's just to grinding on the ears.
Next track Peter Gabriel's track Red Rain of the remastered So album -
Beyerdynamic DT1350 - Bass is there again as is that nice controlled midrange, no struggling here or mashing of ears, good separation of the hi-hats here, that piano disappears into the distance, oh never heard that sound in that place before! Two minutes 40 into the track the vocals are good, controlled not strained they are there to be enjoyed. Coming to the subtle end, bit of confusion there with all that banging and crashing of drums but the vocals are still good, that reverbed piano is nice., great separation at the end.
HifiMan Re0 - Toy town sound just where is that bass and midrange, vocals straining here, upper mid treble detail better as previous, but God these vocals hurt on the ears, must persevere... Seems more congested a lot less separation here when things are busy, mmm some little details again there showing through... Coming to the two minute 40 mark, oh its not that bad, but it's certainly not as sweet and as controlled as the Beyers, but it's all too bright and annoying. OK subtle ending coming up, recessed vocals here, mmm what are these headphones doing, tinny sound here at the end of the track, no lower midrange sound here, it's all in the upper mid / treble area and the mid string noise is very lost and not half as apparent as the DT1350.
Conclusion - No competition the Beyerdynamic DT1350 is head and shoulders above the HifiMan Re0
Before I did this comparison I believed that the Re0 headphones were quite a good performer, whilst they do stand up to all the observations about the amazing amount of detail that they can reproduce it's a small benefit to the overall sound that they possess, they really are quite painful to listen to compared to the more controlled Beyerdynamic DT1350.
Where I could quite comfortably sit here all day listening to the DT1350, I struggle putting in half an hour with the Re0's, admittedly the DT1350 does have it's faults with that not Earth pounding bass and not revealing the more subtle details in productions, it's the overall sound that matters and in this there is no comparison, which is hardly no surprise considering the cost differences in the products being reviewed...