My local GC doesn't have the DT880 or x70x (or at least they didn't three summers ago when I was last in there). If they did, my search would have ended sooner....
It's really variable how helpful staff will be pretty much anywhere you go. The time I was in that GC when I ended up buying the M50, everybody was really helpful, pulling headphones down from the rack and getting them out of the box for me to try. When I went back to return the M50 a different guy made it clear (without saying a word) that nobody would be pulling down any boxes for me this time. He was almost chiding me when he pointed out I'd have to pay a restocking fee (which I already knew). I get that shrink wrapped stuff can't be tested (like the old-style, gold-trimmed K240 they had which that I was sorely tempted to just walk out with sight unheard for $119), but still. Who would buy headphones without at least trying to hear them first? I don't see it as too unusual a request to make.
They didn't have an awful lot, anyway. From what I remember there were the M50, SRH440, SRH840, MDR-7506, HD280, DT770 (the exclusive version GC used to sell), the K240 I mentioned, and the M40fs (I probably should have tried those the first time). Not a terrible selection for the time (summer 2010), but nothing really exotic, either. Also, nothing open-backed, interestingly enough. I had expected they would at least have a K70x, HD600/650, and/or a DT880/990 to try.
You're lucky you have a store with anything. My nearest store is HMV which is like 10 miles away and It's full of beats and low end Sony and Sennheisers