I was always a skeptic about this option in Rockbox. Recently i was bored, and i gave it a try. And that was it. Crossfeed, gives you the most realistic bass experience you can have using headphones. With it, i could have the missing boom effect. The impact was there. If you complain about the lack of bass while using headphones, give it a try.
Notes: When enabled, crossfeed tends to lower some frequencies, so you might want to equalize to compensate for that.