Sennheiser really managed to create 2 impressive single DD IEM ...
I really liked the IE900, and it's probable the most comfortable IEM I ever used.
But the IE 600 is so fun with those "spicy" trebles while keeing a quite nice balanced tonality, that I think it should become soon or late my most used IEM beside the DCA Stealth Headphone and I won't regret the IE900 to be out of order.
The Vision Ears EXT will probably stay as my preferred IEM for more critical music sessons, trebles are nearer "reality", and I dream of an Hybrid Sennheiser IEM to improve a bit more the details of the High frequencies of the IE600
Unfortunately the real king IER-Z1R has no support in Europe as it's not really available, and mine was bought on ebay and is out of order after using it 2 years. But the IER-Z1R is far to be as comfortable in the ears as the IE900/IE600 (those are really a perfection).
But probably in the near future, my main use of the Vision Ears EXT will be with a Nayparm modded WM1A.
And the main use of the IE600 will be with the iFi Go Blu
Both DAP/Amp are mostly used as BT receiver from my Samsung Galaxy Z fold 3 + UAPP + Poweramp EQ + Qobuz + SSD 4Tb (DSD+Flac for Planes/Trains) ....
Unless there will be in the near future on the market a Sennheiser formfactor IEM with hybrid drivers like IER-Z1R/VE EXT), I think I will stay with the IE900 or IE600 for a long while now.