SM7B through a dbx 286s into a Scarlett Solo: which input does the 286s go to?
Into the 1/4" jack with LINE selected, never the XLR — and the Scarlett's own gain goes to zero. The 286s supplies up to 60 dB where the microphone asks for 57, so the interface preamp has no job left.
Short answer: microphone → XLR → 286s → balanced 1/4" TRS → the Scarlett's instrument/line jack, with LINE selected and the interface gain at minimum.
The arithmetic explains why. Speech at 85 dB SPL leaves the SM7B at about −65.8 dBu; the Solo 3rd Gen clips at +9 dBu, so a −18 dBFS target needs 57 dB. The Scarlett's own preamp maxes at 56 — one decibel short, which is exactly why this pairing gets a channel strip in the first place. The 286s offers up to 60 dB. Once it is in the chain the requirement at the interface is zero: the 286s is not helping the Scarlett preamp, it is replacing it.
So the Scarlett's knob belongs at its stop, not "somewhere in the middle". Two preamps in series do not share the work; the second one amplifies the first one's noise along with the voice.
The detail that trips people up: the Solo's second input is one physical jack with two personalities, and the personality is chosen by the INST button on the front panel — not by which cable you plug in. Leave it in instrument mode and you present a high-impedance, unbalanced input to a +4 dBu balanced line output; it will be hot, unbalanced, and you will hear the room's electrical hum. Press the button so the input is in line mode.
One more thing worth knowing before you buy: the 286s' compressor, de-esser and gate are printed into the file. Unlike a plugin, a gate that clipped the start of a word cannot be undone later, and the SM7B is quiet enough that the gate threshold has to be set against the microphone's own noise floor rather than the room's.