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.
If listening to something like a studio multitracked recording of rock album then the differences are smaller and that unbridled resolution isn't really needed and can actually be a detriment.
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.