In my jazz bass sheet music there is this ballad and it shows a series of descending sixteenth notes with a large curved line above them. In brass music that would mean to slur it. But on bass the range is too great to slide or pull off, so what am i supposed to do? Is the line just to make everything even or what?
Play it as legato as possible. You might have to pluck a few of the notes, but try to do it as little and as softly as possible.
Only play what you hear. If you don’t hear anything, don’t play anything.
-Chick Corea
And the song is written at 60 bpm, but there is a part in the middle that says double time feel. Do i play 120 at that part.
Yes, if it says to double then its 120 in that part.

On that other you said it means to play all those notes like if they werent separate, without leaving spaces between them and just let it flow c: Oh yes, some good bass hehe
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It means flat dynamics, no accentuation of notes landing on rythm beats.