I can't tell the difference between speaker cables at all (yet). I don't know why this is so different for me with headphone cables, but it could be due to how I only have $200/each JBL Northridge speakers.. I also haven't been able to yet tell the difference between interconnect cables I own for my amps. Some Cardas cable (I got for free) and Monoprice cable. I plan on making some new interconnect cables, so I'll try again.
So my point is that I never could tell a difference between cables in the past until I changed headphone cables. Weird huh? I've become such a fan of this cheap Mogami stuff I might see if I can make some speaker cable out of it and see how it goes.
For my speakers right now I'm using some thicker gauge blue wire I found in the auto section. Mostly because two years ago everyone told me there is ZERO difference between it and the expensive Monster Cable Sounds fine to me.
BTW based on my experiences, most of the stock cables are not very good. K702 cable isn't bad and the HD-650 cable isn't as bad as I originally thought. The HD-598's stock cable is just dreadful compared to the cable upgrade I did. If anyone heard the 598 with Mogami they wouldn't be buying the HD-600 I wish I was making that up.