To start, the 909s are AMPs picky, so picky some of your best amps will not pair well with them while your lower cost amps will make them sing, this is all due to the low impedance that they have. If they are not paired well, you'll find the highs shrill and feel like daggers through your ears and the bass will sound flabby and out of control. However, find the right amp and sit back. Clarity, low end, transients and layering are all TOTL level.
These are amazing with the subwoofer like bass that digs deep, resonates and has a slow decay. It never affected the mids which were wonderfully clear along with non sibilant highs, however, the sound stage might not be as big as the Senns 800s, but it's wider than similarly priced headphones and has a 3D extention in all directions.
Overall, they're very very recommended. Plug them into everything you have, I found that they worked best with the Chord Hugo TT2, iFi Micro Black Label, PS5 controller (yes and they sound amazingly good!) and the creative soundblaster X 6! Happy listening!