get TSMuxerGui (google it, it's free). load the mkv into TSMuxergui. select either ts or M2ts as an output. now you need to click on the video stream (should be stream #1)and check its profile level. If it says 5.1 then you need to change the level to 4.1. This doesn't work all the time so your best bet it to use the split & cut feature and output a minute or so of footage that you can test first on the PS3. just stick it on a USB stick or external hard drive. If this works then your good to go. If you get audio and no picture then it has an illegal video stream and would need to be re-encoded. Now you can output your new file. It will only take a few minutes. The file size will be slightly larger than the original but if its only 6.55gb then it will fit on a DL-DVD.