Yes, you can use a Beyerdynamic DT 990 Pro with 250 ohms with this device, but the volume may not be optimal. The integrated XLR interface offers up to 75 dB gain, which is sufficient for many headphones, but with high-impedance models such as the DT 990 Pro, the maximum volume may be limited.

It's with the extension! However, I wasn't sure at first either. This could be specified more precisely in the product images and product description.