In my mind; it seems obvious that the way Heighten Spell would work with other metamagic feats is that you would first heighten the spell to the DC level you desire and then add the metamagic feat. For example; if I wanted to cast a Heightened, Silent Fireball at a 5th level spell DC; it would be a 6th level spell: Heighten Spell to 5th level and then apply the Silent Spell feat raising the spell level to 6.
In my last game session, it was suggested that it should not work this way and that you could use the Heighten Spell feat to raise the DC of a Silent Fireball (4th level) to that of a 5th level spell for the cost of a 5th level spell slot.
When I did a forum search for Heighten Spell, there was an archived thread where one poster played it using the first method and another poster played it using the second.
Would people mind chiming in on how they would rule this combination, please?
Thanks!