@archangel.arcanis: I'll do my best. If I was more eloquent in my wordsmithing, everyone would already know how I arrived at my position -logically - on this issue. My apologies, everyone.
Yes, I know I've posted this before... but here it is, one last time.
HEIGHTEN SPELL [METAMAGIC] -emphasis mine.
You can cast a spell as if it were a higher-level spell than it actually is.
Benefit: A heightened spell has a higher spell level than normal (up to a maximum of 9th level). Unlike other metamagic feats,
Heighten Spell actually increases the effective level of the spell that it modifies. All effects dependent on spell level (such as saving
throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level. The heightened spell is as difficult to prepare and cast as a spell of its effective level. For example, a cleric could prepare hold person as a 4th-level spell (instead of a 2nd-level spell), and it would in all ways be treated as a 4th-level spell.
Everyone has no problem with the 1st bolded point. Clearly Heighten allows spells to be, well.. .heightened no higher than 9th level.
The second bolded statement everyone ignores. This is apparently because it has the phrase 'prepare and cast' in it. When thinking about DMM, everyone says 'well, that doesn't apply'.
Here's the statement again:
The heightened spell is as difficult to prepare and cast as a spell of its effective level. Let me ask this question: What is the implications of this statement?
i.e. what rules must logically follow from this rule, that haven't been spelled out in actual text within the Heighten feat, but exist elsewhere and still must apply to Heighten? Ignore DMM for a moment.
1. The character casting such a spell still must have a casting stat that is high enough in order to do so. Ex: Cleric casting a 9th level spell requires a 19 WIS.
2. The character casting such a spell must have a spell slot available of the appropriate level. That maximum level is further dependent on the character's abilities with regards to such. Ex: A 5th level Cleric can heighten a spell to 3rd level, but not to 4th level.
3. Item 2 means that even though the range for Heighten is given to be 1st through 9th level spells, the
effective range for a 5th level Cleric is 1st through 3rd. A 5th level Cleric with Heighten can not 'set' (for the lack of a better term) the spell level increase above 3. This is a restriction enforced on the character by the 2nd bolded statement within Heighten. This restriction is true for any prepared spells. And it's true for any cast spells. Both prepared and spontaneous magics are covered here.
Now factor in DMM:
4. DMM lets you spend turn attempts in place of the bump in spell level that Heighten normally requires.
5. DMM can not modify the spell level cost of a metamagic feat. It can't change a +3 to a +9, for instance.
Now factor in both:
6. Heighten has a maximum spell level increase AND a restriction on a characters maximum spell level increase. Regardless of whether the spell is prepared or spontaneously cast.
7. For a 5th level Cleric, a spell can only be heightened to 3rd level. No matter
how its spell level increase is paid for.
I hope that this clears up at least the question of how I derived my position. If not... Hi Welcome.