Thanks, but I wasn't looking for an external utility to calculate replaygain and convert it to soundcheck tags. Those are readily available from many sources.
I want an app, on the device itself, that would be able to read replaygain tags that can include both track levels and album levels. In a mix I'd use track levels, on an album, clearly, I'd opt for album levels.
The current soundcheck option caters *only* to people that listen *only* to mixed tracks. Otherwise it makes a mess of your albums.
On Android there are several options and, actually, if there is none on iOS I might consider switching. The different volumes are supremely annoying to me.