Only when the name of the .srt file exactly matches the video filename, it identifies the subtitles properly. I mean even if there is a difference with a space ' ', would not properly identify it.
So generally what I do is, on your pc,
F2 the video file, (rename)
copy it,
F2 the srt file, (rename)
paste it.
I do this for my WDTV to identify the subs. The same applies to anything, as this has become the standard.
Also for Philips 5609, seems there is an option as subtitle zoom. try that on the remote or settings menu. So, surely an option would be there to switch it off, when there is an option for zoom.