Will a Scarlett Solo drive 250 Ω DT 990 Pro headphones?
Loud enough for everyday listening, with little spare for peaks. The Scarlett Solo 4th Gen reaches about 108.2 dB on the DT 990 Pro 250 Ω. Published specs.
The Focusrite Scarlett Solo 4th Gen will play the Beyerdynamic DT 990 Pro 250 Ω loudly enough for most days. What you do not have is a comfortable reserve when the music suddenly gets much louder.
The DT 990 Pro 250 Ω is 250 Ω. Reaching a 110 dB peak — a loud moment in film or classical, not a casual average — takes about 2.51 V from the source. The Scarlett Solo 4th Gen gets to 108.2 dB on this load.
Separately, the output impedance is 50 Ω into 250 Ω headphones. On a dynamic driver that ratio can lift or dull the bass. It is a note about tone, beside the loudness result, not a replacement for it.
On compressed pop you will not notice. On classical or a film mix the volume control will sit high. Buy a headphone amp if you want those peaks clean. Do not buy one because someone said every 250 Ω headphone is impossible from an interface — that is not what this pair does.