I tried versions SB1/SB5/SB6/SB7/SB8/SB9/SB46 of Mplayer+Mencoder archives.
On versions SB1/SB5 there is not subtitles displayed at all. (??)
On versions SB6/SB7/SB8/SB9/SB46 the subtiltes are displayed, but the "POS" tag of the srt is not understood ("POS" instructions are displayed as text).
My test method was :
movie file + srt file are in the same directory, with same name (except extension)
movie file drag/dropped on the mplayer.exe of each uncompressed Mplayer+Mencoder archive
Seems like this method can not help to identify the problem, because the Mplayer+Mencoder archives' behaviour is different from PMS.