when I practice seated, I play with the bass between my legs and my left foot higher like a classical guitar stance. When I play standing my bass is in pretty much the same position. The classical guitar position is also great for good hand placement. I used to have tendonitis problems when I first started until I read up on classical guitar finger stretching and playing position in a very old book. I've played that way ever since and never experienced any more hand pain.
It's also important ( for me at least ) to have a wide and stable strap so that my bass fits me securely and I'm not trying to "hold" my bass with my hands while playing it.
Ps. I'm an exbodybuilder with a large chest... I can see the fretboardmaybe try a different strap position.
Best of luck.
This is pretty much what I was about to type. I usually practice standing up but when I sit I use the classical guitar player position so that my bass is in the same position whether I'm sitting or standing. When the bass is constantly in a postion that is comfortable whether you're sitting or standing it makes everything else so much easier. It really makes the difference between playing the bass efficiently or having to fight the bass to play it even for the simplest of lines and techniques.
