C4/Record.pm marc2bibtext now has: title => $record->subfield("245", "a") || "", We would also like to include 245b subtitle here.
Note that for the newer biblatex there are also separate fields for subtitles. (`subtitle` and `titleaddon`). I think 245__b should end up in `subtitle`, `titleaddon` is more for descriptive notices (e.g. `Proceedings of the...`) In any case one is left with some incompatibility between legacy bibtex and biblatex here. It might be the user friendliest way to go for the separate `subtitle`-field of biblatex as it's easy to join it with `title` and the necessary separator char for those who use legacy bibtex and it involves less typing to remove than to add, and it would not "break" biblatex compatiblity as well.