[QUOTE="TheGrooveDude, post: 17380373, member: 247771"]How did you manage to take the thumb out when angling your bass? Are you sitting?[/QUOTE]
...Dude,
As a long-time "Stander" (37yrs.), I'd like to offer another approach to the "thumb" issue. I believe that the thumb should not be "taken out of the equation", (sorry Chris), but rather, should be included and involved in the total system from fingertips/knuckles/thumb/wrist/elbow/shoulder/chest/back/etc. If any portion of this path is tense/tight or inflexible it will lead to fatigue, tightness and even pain.
First, Geoff's (standing) approach is spot-on in terms of having the bass "lean" toward the player, as shown in his excellent video. I would encourage you to experiment with this concept, even to the point of NOT having the left-knee touching the back of the bass. I was taught to stand normally, feet slightly apart, with an even distribution of weight - not favoring or bending either the left or right leg. This ensures an even distribution of weight as well as a healthy posture. (It does take some getting used to - especially if you've sat in the past, or if you've bent your left leg in order to touch the back of the bass).
I would encourage you to keep the thumb (generally) under the fingers, in the classic "C" shape, not pointing back "hitchhiker" style, (which may work better for the "Sitters"). I believe the goal here is to use just enough energy (between the finger(s)/above and the thumb/below), to get the string to sound. Much like catching a ball (or an orange), you just need to use enough energy to make the catch, NOT make dust, or juice, (or worse, like "Lenny" in Of Mice and Men!). There is a tendency, especially in the beginning, to use more energy than is necessary, which can lead to the dreaded "clamping" effect. I would guess that if there is discomfort and or pain in the thumb, it is probably indicative of rampant system-wide tension and inflexibility throughout the left-side of the body, NOT just the thumb. (Enough "Thumb-Shaming" already!). There are exercises and strategies which can help identify and isolate these areas of tension and rigidity.
I was trained in the "Simandl" method for the first 4 years of study, which was a very valuable system of organizing the left-hand. I could play in tune, (generally), but my left-hand was somewhat "tight", and my shifts were not smooth. My second teacher (J.B. VanDemark) really opened me up to a more flexible approach, especially with the introduction of Vibrato (Arco). He had explored and was influenced by a "cellistic" (is this a word?...It is now!) approach, (as a student of cellist Paul Katz), beginning with the fingers being "non-perpendicular" to the string, slightly tilted back, (VERY Un-Simandl), and not keeping the rigid, 1 - 2 - 3,4, shape, except when necessary. The introduction of Vibrato study, with the bow, really exposed the inherent tension and rigidity in my left-side, and led to a more relaxed, efficient, and ultimately stronger left-side.
Thanks.