MKVToolNix and subtitles

Hi, I have a router with a dlna server built into it but it can't handle the streaming of external sub files. The solution that I was suggested was to convert the files with MKVToolNix to mkv and merge in the sub files. My concern though, is - that if I do that and simply recompile all my different files from their original formatting to mkv I will loose quality of the video or audio. My question is - is there any data lost during the recompilation process and if this is the perfect way to achieve my desire? also - do you know of a better way to do so without the need to change the file format? 
 
Thanks

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