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If you're slouching you're not doing it right. You need to bend your knees and bend at the waist, not with your back. Let the bass fall "into" you instead of you bending into it. And try not to look down too much at your fingers. Your head is a heavy object and tilting it downward puts pressure on your back to carry all that weight.
+1 on the Alexander Technique suggestion but even before that... GET A TEACHER. A double bass specific teacher at that. If they're worth their salt they will tell you what you need to do for your form.
Getting stance/form suggestions over an internet forum sucks eggs.
+1. One of the most used phrases on this side 'Get a Teacher' works extremely well. There's several things you could be doing, but we don't know for sure without seeing you play. If you can't afford a pro player, find the local music school & find a graduate student.
Whenever I start to hurt, it's a reminder that I'm being lazy and need to correct my posture/technique. For me, the 2 main things that I do often are slouch and lock my knees. Either of these leads to a sore back.
A good option for me would be to get a teacher and learn the proper way to play on a stool since the main times that I slouch or lock my knees is during theater gigs.
all the best & good luck with it.