Right so I browsed the web some more and today found a solution. I have to say that the subtitle quality is better in KMplayer then it is in the standard media player. But here is what I did:

What you have to do is install this file on your N900. First place it in the root-folder of your N900 and make sure you have Rootsh installed to gain rootacces. Then install the .deb file via X-terminal. If you do not know how to do this, there is a guide here and I have added some screenshots below. After you have installed it, go to settings and at the bottom at "Extra's" you have the option "Subtitles". Open it and click "Automatically load subtitle files". And you're done. Make sure the videofile is named the same as the subtitle file. Except for the subtitle file having the file-extension sub or srt.









Good luck and let me know what you think.

(I am ofcourse not responsible for the possible bricking of your phone, just telling you how I did it.)