Sorry - only Sony's I've heard were very cheap portables - many years ago. Have not had the pleasure of listening to the MDR7506 or 7509 - will have to correct that at some stage
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My DT880 were the Pros - 250 Ohm.
My impressions only - others will have different viewpoints.
Build - both 880's and 600's well built - 880's would be more durable.
Comfort - both
very comfortable. Both were quite clampy at first (the 880 pros clamp harder than the premiums), but both settle after use. I would have no issues wearing either for long sessions. The Beyers have the slight edge on comfort, but the Senns are not far behind.
Isolation - similar on both - they're both pretty open, but I found less leakage out than something really open like a Grado.
Sonics - similar in that they are both relatively balanced cans - without strong emphasis on any frequency.
* DT880 has slightly elevated bass and highs, and definitely has more extension top and bottom. It's pretty good with faster paced music. Bass hits really nicely. Mid-range can be slightly 'cool', and to me almost slightly recessed - but think this is more due to the little more forward bass and highs. Mid-range can be brought forward with a warmer amp - and pairs very nicely with tubes. Gives the impression of more detail - mainly I think because of the more forward highs.
* HD600 is slightly more relaxed, and to me a little more 'intimate'. A bit warmer - but not necessarily dark. Strength for me is in the mid-range which is slightly more forward. Doesn't extend as well top or bottom as the DT880, but bass is tight, balanced and has nice impact (when required). Clarity is really nice - and to me has more sense of dynamics / realism than the DT880.
Of the two - I prefer the HD600 - especially coming from the SRH840 originally. It think it's the mid-range. Both are very natural and balanced sounding cans - with the HD600 being more natural to my ears.
Both respond well to amping - and the HD600 continually amazes me on tracks as to how well it reproduces timbre and dynamics. I was listening to a live performance of Nights From The Alhambra (Loreena McKennitt) last night, and so far it's the only headphone I've heard (I haven't heard flagships yet) where I can close my eyes and I'm sitting in the audience. The gentle wash of applause just took me by surprise (again). I didn't get that with the DT880 - although the 880 is a headphone I miss already, and may buy again. I will not sell the HD600 though - ever - it is just too good.
Hope this helps. WIll be interested to hear your impressions also.
Main listening with both DT880 and HD600 was out of an NFB-12, and/or PortaTube. Eventually I'd like to step up my source and amp - but I do find that my current sources are more than adequate to keep me very satisfied.