IMO you own a quite good headphone-setup. Before you think about the very expensive STAX-Upgrade I'd abdvise, to inform you about the smyth realizer (which is also an expensive investment). When you once heard the difference of the sound in the middle of your head (most headphones) and a binauralized sound, you'll never want go back.
Because of my little apartment I have no opportunity to get a speaker-rig and my neighbors would stop loving me
In future I also want to invest in higher end (maybe also in SR-009) but at the time I'm satisfied with my HE-6/EF6 combination in partnership with the smyth realizer.
If you want a first feeling of it, try the examples of darinf in the "Smyth SVS Realiser - PRIR Exchange Thread". Even if the measurement is made with his ears you can get a good impression of the capabilities hearing with headphones.
Give it a try.