Converting videos with ASS subtitles

[Kracov]

Does anyone know another program other than AVC that can properly convert subtitles with backgrounds, namely the ASS format? AVC fails when trying to convert from MKV to MP4 (any format). Some MKV videos have indexing problems that can't even be fixed by the mkvtoolnix program. If you need the media info and the logfile, I posted them here https://forum.videohelp.com/threads/395052-Problems-converting-MKV. But ultimately I'm just looking to use a different converter. Xvid4PSP and Handbrake seem to handle the MKV videos fine but no background box. Xvid4psp doesn't hardcode the subs.