Rx: Improve intonation in the area below the octave, just before getting to thumb position.
Geoff Chalmers shows how to figure out which notes are where based on the physical construction of your bass.
This video is from Geoff Chalmers’ DiscoverDoubleBass website. While he is focused on jazz playing, there are several lessons on his site that are easily useful to anyone playing bass whether they are into New Music or any other kind of bass playing.
I also recommend using the Tom Gale book in conjunction with the stuff that Geoff is talking about here. A great combination with classical-oriented etudes and exercises for working through all this.