I am not sure about the SQ difference. But there is a very big price difference with Stax 009 setups costing from $7000-$12000. If you have that much money to burn, then it seems to be one of the best HP systems.
Of late, one headphone that reviewers seem to be comparing to Stax 009 is the Abyss AB-1266. It seems to have the best of HD800 (imaging) and the LCD-3 (Bass and easy to listen). It's of course $5495 and seems to need an Amp as powerful as the Taurus Mk2. I plan to borrow it for a while to see what all the fuss is about.
Personally, I have the HD800 and the LCD-3, which complement each other very well. HD800 for Orchestral and Big Band music and LCD-3 for all my other genres.
Lol our posts are like seconds apart, and like you also, I am loving my hd800+lcd3 combo.