Although ive only been playing in thumb position for a few months,
1) in the begining i never had pain in my wrist or fingers because my teacher took an entire lesson to show me how to use the position
2) i havnt developed the habit of wresting my arm or wrist on the bass when in the position
3) to solve the case of the terrible tone, remember that as you go up the finger board, your bow must shift as well, ask your teacher to show you how its done, you dont want to develop swimmers bow.
4) Also very common with poor bow tone is the angle of the bow and the stick. The bow must remain parallel to the bridge, and on the G string it can be difficult, and the stick should be slightly angled towards the scroll.
This is a lot to think about, you should be practicing this in front of a mirror under teachers instructions, dont just go and do these things, you will develop bad habits, and these can physically hurt you.
w w w. y o u t u b e . c o m / u s e r / N i c k F e i g e n b a u m B a s s H Q