I just received the same advice from an "Applied Kineasiologist" in the Boston area and he more or less said the same thing to me among other things.

He noticed that my left shoulder, on the same side I'm seeing TOS symptoms, was much higher and more forward than the right shoulder. He then went on to poke and prod my muscles and told me that my left pec minor was super tight vs. the right. He even put my finger on each so I could see the difference. Same goes with the muscle just beneath the clavicle. These are two key areas of compression. These two muscles are pulling the shoulder bone forward creating less space in the thoracic area.

He then went on to test my back muscles. Compared to the healthy right side, my lats, rhomboids, and infraspinatus were all weak. If healthy, these muscles would pull that shoulder bone down and back into the socket...also creating more space in the thoracic area.

His instructions were to massage the tight areas for 2-3 weeks. NO STRENGTHENING YET. Then, focus on strengthening the lats and rhomboids.

Not sure if anyone else is in my boat, but it's worth a shot if you think you are tight in front, and loose in the back.