There isn’t a way to adjust the positioning unfortunately. You may want to try and obtain an alternate SRT file for the affected movies.
nitoTV has 2 playback modes. Mplayer and QuickTime. You can switch between these two modes in the nitoTV > Settings menu. Most files are best played in Quicktime.
Naming of SRT files is important. The SRT file should have the same name as the movie file you wish to play it with. Also, make sure both fies are located in the same folder.
EG
Media file: TheOffice.avi
SRT file: TheOffice.srt