Does your music have an initial tempo setting at the beginning? If so, I think that takes priority. Remove that and then you can adjust playback to whatever you want. There are also settings in Expression Designer that you can make to ignore that initial tempo mark, I think.
It does have an initial tempo setting and it SHOULD have taken priority, but it was ignoring that and using the greyed out tempo - that's why I noticed it was greyed out - I needed to make that match the tempo marking. There is something strange going on, because the same piece when opened in Finale 25 on my laptop behaves absolutely normally - it has an initial tempo marking but I can also adjust the metronome mark in the playback window.
The first 'stop-button' solution fixed a big frustration for me. Thanks! But I agree that there should be a better, more intuitive, more efficient way to set and control tempo. Maybe the new version? Next?