Probably the best legato player is Allan Holdsworth, and he doesn't use pull offs (in the traditional sense).
Instead of fretting two notes on the same string and then pulling the higher finger off to sound the lower one (as most people do) he frets one note, removes his finger from the string and then hammers on the lower note.
I've tried this and it's very tricky, but I can imagine it would sound much smoother. No scr@ping callus on the string or nasty TWANG if you pull too quickly.