Can anyone recommend a device for playing videos to a smart TV?

Unfortunately I don't have a Samsung TV but Sony, and Sony's are not as 'smart' as Samsungs for reading MKV files and allowing you to use SRT subtitle files

I just need some device that can read both MKVs and use SRTs - what are the options? I have my content stored on a Synology NAS but I believe my NAS (DS 213Air) is too weak for transcoding.

Just wondering if something like Apple TV, Amazon Fire TV, Roku etc will do this? OR maybe even a Pi2? Cheers

I've got the Amazon Fire TV box, and you can easily download the Plex app to it. Plus you can sideload Kodi to it fairly easily as well (doesn't require any rooting or anything).
http://kodi.wiki/view/HOW-TO:Install_Kodi_on_Fire_TV

I use Kodi to play movies on a hard drive connected to my router, and you can certainly turn subtitles off and on with MKV movies.
Not sure if they're the right format of subtitles for you as I never use them myself , but you probably know better than me if Kodi/XBMC is suitable for you anyway.