Unless I’m doing something wrong, it appears the bass xylophone and bass metallophone ranges are incorrect in sound and notation.
In Dorico Pro 3…
The lowest notated that makes a sound is 3rd space C on the treble clef for the bass xylophone.
The lowest notated that makes a sound is G on top of the treble clef for the bass metallophone.
Edit>Transposed Pitch & Edit>Concert Pitch has no effect on the notation.
The bass xylophone and bass metallophone are supposed to be notated from C below the treble clef to G on top of the treble clef.
They sound at least one octave lower.
In their current state in Dorico Pro 3, these instruments are unusable. (Regardless of notation, they sound few or no pitches that are actually in these instruments ranges.)
My present solution is to use a xylorimba for the needed notes. I don’t yet have a substitute for the bass metallophone.