Only one of two DAC are used for SE, regardless it's noticeably louder than the BTR3.

That would work, so would this Startech cable from Amazon  

I got my cable from Winners for $7.99.   I don't think that would work (Hosa cables are designed for connecting up studio components and probably have full-sized cable ends)... suggest that you buy in-person from a brick-and-mortar. Long & McQuade (Canadian musical instrument chain) probably...

Yeah most definitely. Based on what I've read about the Pro80 though you may notice less mid-range in comparison? Those are designed to be more "neutral" flat, while HD5500 are designed to drop a bit of upper midrange and boost the highs (see the sound signature graph here, can't upload the pic...

Sound leakage on Takstar HD5500 is minimal... definitely won't both your fellow commuters on the train, or your GF sleeping on the couch a few feet away from you :-)

You won't go wrong with the Takstar HD5500s... very comfortable and sub bass for days. The good Chinese brands put the ish you find at similar prices at Best Buy and the like to shame. 

I'm finding the Takstar HD5500 to be excellent for EDM, clarity better than any consumer brands I've tried (especially when paired with a cheap DAC/Amp like Fiio E07K), and have the most comfortable pleather pads of any of the cans I own.

Had the Takstar HD5500 for a few weeks and was mildly underwhelmed... nice sound but quite the "V curve", lacking in midrange presence I like.   Paired these puppies with a Fiio E07k USB DAC / Portable Amp and their potential truly shines...never had such sub-bass extension and clarity in a...

I bought a cheap 3 foot fabric cable at Winners (Canadian TJ Maxx) that works perfectly.... basically you need one end to be thin enough to fit.    Mine is branded "Urban Beats" but looks exactly like the "Merkury Innovations 5 Foot Two Tone Braided Fabric 3.5mm AUX Cable".

Just want to report I picked this up the other day and was surprised to find that the USB DAC works impeccably with both my Nexus 7 v2 and my Bell Samsung Galaxy SIII with Cyanogenmod 11 (Nightly 2/26/14).    Almost went with the E18 at a $50 premium for "android compatibility", glad I didn't!