You should be able to stand perfectly straight with the bass, as if you were just standing normally. The spine should be straight and perpendicular to the floor. There should be no strain on the spine.
The reason it is better to listen behind the bass is that with your head in front of the neck and over the top, you are mainly hearing what is coming directly out of the instrument whereas by keeping your head back, you are better able to hear the sound more as it is in the environment (be it a performance hall, practice room, etc.) You want to try to hear what others will hear, not what you can hear a foot over the top of the bass.
(I'm sorry if this is unclear. I'm really bad at wording things.)
The reason it is better to listen behind the bass is that with your head in front of the neck and over the top, you are mainly hearing what is coming directly out of the instrument whereas by keeping your head back, you are better able to hear the sound more as it is in the environment (be it a performance hall, practice room, etc.) You want to try to hear what others will hear, not what you can hear a foot over the top of the bass.
(I'm sorry if this is unclear. I'm really bad at wording things.)
