People sit behind it, most don't stand behind it, Unless you use a Bent/Laborie/Rabbath Endpin which i highly recommend.
Here are some of the advantages of being behind it:
1. Your bow technique dosen't have to change, and you don't have to shift your bass or your position to play the E or G strings.
2. You can keep your wrist straighter.
3. You can sit or stand and the bass basically rests the same way.
Before i got an angled endpin i sat behind the bass as i wanted to facilitate the technique in that respect as much as possible.
As far as the low end, perhaps it is a change in the technique. Honestly without some scientific equipment and a snapshot of how you were playing it before you transitioned and after it would be hard to tell. Realize that you may now play different because of the change.
"The Intonation is evidence of a Correct Motion."