If it's really as thin sounding as you say perhaps the pickups just need a little tweak. The first thing I would do is raise the PU's a little. I haven't heard anyone else saying these are thin sounding so I'm questioning the fact that yours is. Pickup height has a huge I pat on tone and perceived power. I think you're a little hasty in your decision to immediately send it back. A little tweak isn't going to void the warrantee. I never make a decision on whether to keep a bass without playing it for awhile. As always, YMMV and all that.
Thanks for your comment, Kirk!
I did check the pickup height and found it to be close enough (around 3mm), so i did not try to adjust.
Following your advice, i did keep the bass for about two weeks, before sending it back to Thomann.
I guess I'm learning that I'm a Jazz Bass sound type of guy. Not a big fan of the physical shape and weight, but I just happen to love the sound.