AVSub 19.5 just hit the App Store.
Only 1 feature in it - crossfade. With it you can not only turn it on/off, but specify the crossfade duration and also choose to either have it on always or just when shuffling.
I've also fixed a bug that could cause the audio to stop while it was fetching cover-art. That doesn't happen now, although there does appear to be an issue with the lock screen. Will look at this for the next release.
Been busy at work, hence the slow updates!