I'm a bit unsure as to what is the prettiest, but these lines really boil down to:
_subtitles = _subtitler->loadSubtitles(_filename, subtitleFile); 

Since the if basically transfers it's condition to _subtitles.

I also suggest changing _subtitles to either _haveSubtitles or _useSubtitles (the first might be better, as the choice isn't up to a user-selection, but up to them existing as far as I understand....