Fostex T50RP mk3 on a Scarlett 2i2 4th Gen: does a planar need its own amp?
Loud enough for everyday listening, with little spare for peaks. The Scarlett 2i2 4th Gen reaches about 106.8 dB on the T50RP mk3. That is the practical answer.
The Focusrite Scarlett 2i2 4th Gen will play the Fostex T50RP mk3 loudly enough for most days. What you do not have is a comfortable reserve when the music suddenly gets much louder.
The T50RP mk3 is 50 Ω. Reaching a 110 dB peak — a loud moment in film or classical, not a casual average — takes about 1.78 V from the source. The Scarlett 2i2 4th Gen gets to 106.8 dB on this load.
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 50 Ω headphone is impossible from an interface — that is not what this pair does.