Synth into a Scarlett Solo: mic input, line input, or a DI box?
Line input. A +4 dBu source is about 13 dB too hot for the mic path at a −18 dBFS target, and a mic preamp's minimum gain is not negative — turning the knob down cannot fix it.
Short answer: the line input, not the XLR. If the only free socket is a microphone input, you need a pad — and a passive DI is one.
The arithmetic first. A studio-level source sits at +4 dBu. On the Scarlett Solo 3rd Gen the converter clips at +9 dBu, so a −18 dBFS target wants −9 dBu at the input. Your synth is 13 dB above that before any preamp has done anything at all.
That is the part people misread. A microphone preamp's gain range runs from some minimum up; the minimum is not negative. Turning the knob to zero does not attenuate — it merely stops adding, and you are still 13 dB too hot, which is why the meter sits in the red no matter what you do. The Solo's line mode changes the input's sensitivity, not just its impedance, and that is the real fix.
Where the ProDI comes in: this database lists it at −15 dB. Put it in front and a +4 dBu synth arrives at −11 dBu — 2 dB under target, which the preamp trims back with the smallest amount of gain it can add. So a passive DI is a usable pad. Its loss is the feature here, not its balancing.
And the part that solves the problem people actually have: the reason a synth into a laptop-fed interface hums is a ground loop, not a bad cable. Two mains-powered boxes joined by a shielded cable give the current two paths back and the difference between them appears as hum. The ProDI's transformer breaks the electrical connection between the two grounds while passing the audio, which is why it cures the hum that no cable upgrade ever did. A hum you can hear is a loop; a hum a better shield fixes is interference. They are different problems.