It is all relative to how much disposable money you have. Performance is not linear with the money you spend. I have a Beyer dt 880 and a Senny hd 650. Inside the 150Hz -18kHz they sound as good as any high end headphone. Take a look on the website innerfidelity - measurements section. For some time I contemplated buying Stax SR-009.
The Stax shine across the entire bandwidth - and they play cleaner at higher volume (you can damage your hearing with clean, quality sound
). Now - the THD for my headphones in the bass range is still better than what you get from a 2-3k$ subwoofer so I am happy.
But - if you want something that has the ultimate sound - only stats will do in my experience.
Are the Stax SR-009 worth the money? You bet! (Provided you have that much money in your "Toy's ur us" budget).