Using a different earpad on a Ultrasone will change the sound for the bad.
I've tried out the Pro 550/650 pad on the Ultrasone Pro 2500.
I assume it will cause the same or similar problems for the Pro 900.
Even though the HFI 2200,2400, and Pro 750,900,2500,2900 all use velour pads they all have their own pads.
I think the Pro 2500 and 2900 might be the same just the silver/grey has become black. Ultrasone moved the blue and silver/grey color scheme to black and silver/grey
The HFI 2400 I'm sure uses the same pad style of HFI 2200 just went from orange to black.
The only place I know of to get Ultrasone replacement pads from is Ultrasone's own website Germany and USA webstore.