The goal is to create as soft music as possible.

It might be EQ, tube preamps, saturation, compression, recording on tape, tape delay, certain mic positioning, or something else. I just don't know from where to start this beautiful quest of making the softest music I can.

Might you please suggest some essential tricks that work for you or some fundamental reading?

Also, might there be any research on which frequencies are perceived as being the softest ones?

Thanks a lot!