Bass is a transposing instrument that sounds 1 octave lower than written, so when I see the Eb one ledger line below the staff, I figure it's probably a 78hz Eb2 and the original bassist on the album played it at the 1st fret of the D string (or 6th fret A string, or 11th fret E string). I'm talking like if I'm sight-reading from a mass-market Pop Singer's Greatest Hits vocal/piano/guitar songbook, that type of sight-reading situation.
For example I recently saw some pro-published Johnny Cash sheet music with tons of low C's (2 ledger lines below the staff). Do you think Johnny's bassist used a 5-string on "Walk the Lilne"? Nope. I watched the concert on YouTube, he played all those C's at the 8th fret of the E string. Pop keyboard sheet music is swimming with low C's and D's, but you don't need a 5-string bass to play those, because like I said, bass is a transposing instrument. Those are just our 'normal' C's and D's.
Finally a completely different situation: If the guitarist I'm playing with down-tunes, for example Hendrix and SRV songs in Eb, then I down-tune to match.