To parrot others, you must use the Bose EQ. I purchased a used pair of Series 1 901s in 1972 because the owner said they sounded flat. He wasn’t using the EQ. At the time, I used a Pioneer SPEC-4 power amp, SoundCraftsman Pre Amp/EQ, and dbx 3bx range expander to tweak the music. I enjoyed that combination until about 2018 when I gave it all away to my goddaughter.
I thoroughly enjoyed the 901s.