Double bass clef usage
Musescore's Double Bass instrument incorrectly assigns an 8vb bass cleft. It should be a normal bass clef. The double bass is considered to be a transposing instrument. This instrument's music is written with a normal bass clef, and it is assumed to sound an octave lower than written. While there are several modern examples use the 8vb marking, it is by no means standard.
This is a problem in MuseScore, particularly with writing double bass music that uses tenor clef (a common practice for higher-register passages), because there is no 8vb tenor clef available. The 8vb marking is unnecessary, a point of confusion, causes serious problems with tenor clef markings, and is by itself a visual distraction. It is critical that this be reconfigured before more compositions and arrangements propagate this error.
Traditionally, the double bass is a transposing instrument. Since much of the double bass's range lies below the standard bass clef, it is notated an octave higher than it sounds. This transposition applies even when reading the tenor and treble clef, which are used to avoid excessive ledger lines when notating the instrument's upper range.
I also have a masters in double bass performance from New England Conservatory, have only encountered the 8vb usage on very rare occasions (usually from inexperienced composers printing bass parts using programs such as this), and a quick scan of all of my professionally-printed sheet music indicates no instance of the 8vb version of the bass clef. ;)