David, I have been doing those stretches you described for a few years now, and they are very helpful in relieve discomfort.
I've been reading a lot of great articles about Alexander Technique, written by and for musicians, and am looking into lessons. Simply being more aware of my posture at my day job has made a huge difference this week; adjusting the position of my chair's back, watching my posture, putting a box under my monitor so that I'm not bending my neck to look down at it; keeping the keyboard closer to the edge of the desk so I'm not reaching for it, etc.
I spent an hour and a half or so last night just playing long tones with the bow, looking in the mirror, trying to adjust my posture, standing and sitting on a stool. I have a very exaggerated chest con-caving posture when I play. In addition to seeking out AT lessons, I'm going to spend a lot of time experimenting with my end-pin height, and I'm going to buy an adjustable stool so I can experiment with its height too. The cheap wooden stools from the big box stores are not a good height for playing the bass (at least not for me). I know a lot of players in my area who sit on lowered stools (my teacher of many years included), in a cello-like position with both feet flat on the floor. I had tried it for a while in college, but didn't enjoy carrying a stool to gigs. I may go back to that for a bit of time.
Thanks all! For those who haven't, check out Don H's website. Lots of good info.