I've been working on ways to get the notes from A2 to F#3 to sound solid and in tune. It is especially difficult when I am on a lower note and go up play, say, C3 or a D3. I've tried all kinds of exercises, and they have done some good. My latest exercise seems to really have pushed me into a new realm. I'm almost there. The exercise ( I have mentioned this before) is simply to play 7th chord arpeggios up and back slowly using only the notes from D2 to F#3. I play them first by ear and then check them with the tuner. Example, GbMaj7: F2, Gb2, Bb3, Db3, F3, Db3, Bb3, Gb2, F2. I have a list of about 50+ arpeggios that I play (not all in one day). I do this for 10 minutes a day. I love exercises that hit two skills at the same time.