Sure, diminishing returns, but depending on the type of music you're listening to those diminishing returns or last 5-10% or whatever can mean a huge difference in reproduction. The Stax SR-009 are flat out better than the HD6XX/HD650 and sheer resolution of hearing acoustic music like piano, violin, sacred cantatas, etc brings me much closer to what is on the disc..
Couldn't debate the SQ issues of the two never having heard the Stax, but to be honest I'm not much a fan of headphone listening to begin with. I use them mainly for questions on recording quality as they in general are more revealing of detail. Only rarely use them for music enjoyment as I've found the comfort issue always to intrude at some point.
That's where we might debate, soundstage, what soundstage? I've never heard anything approaching what I would call a soundstage from any headphone.The main issue with the HD6xx/650 is the soundstage, it is very constricting for any natural space/non-studio recordings and second it's not as resolving as the Stax. Dollar for dollar there is nothing remotely close to them.