Sublime Chord doesn't reference "spellcasting level," which would arguably be congruent with "caster level." It references "level in a spellcasting class," which is congruent with "class level."
If you are a Wizard 10, you have 10 levels in an arcane spellcasting class. If you are a Wizard 7/ScM 3, you have 10 effective levels in an arcane spellcasting class. They are functionally equivalent as far as Sublime Chord is concerned.
+1 spellcasting level is
not the same as +1 caster level. Otherwise
orange ioun stones give you spell level advancement, spells/day and spells known. Incidentally, "spellcasting level" and "levels in a spellcasting class" are functionally identical. The only corner cases I can think of would be Kobold Sorcerers with the Greater Draconic Rite of Passage, wherein they are Sorcerer X with Sorcerer X+1 spellcasting. And in those cases, because you are treated as having gained a level for the purposes of spellcasting, Sublime Chord would count the Rite's boost in it's calculations.
Caster level and class level are two distinct values, both of which have specific and clear definitions in the rules, and they're NOT interchangeable.
I suppose you could argue that "level in a class" is not the same as "class level," but I think that's stretching semantics to the breaking point.
As I said: I know it's not a popular argument; the popular argument is generally the one that gives the most plusses. It is, however, the least-liberal reading of the rules.
Ultimately, though, it doesn't matter whether you accept my position, or I accept yours; what matters is what the OP's DM says on the matter.
I'm not arguing that Sublime Chord's CL calculations incorporate +CL (as opposed to +spellcasting) increases. I'm arguing that PrC levels count when calculating your Sublime Chord's CL because they stack with your base class(es) for the purposes of spellcasting.
To use an analogy, look at Turn Undead. The section for Turn Undead in the PHB references your Cleric level for everything. However, if you have a bunch of PrCs which advance Turn Undead, you get to use those classes when calculating your Cleric level for turning undead. The exact same thing is going on here. For a bunch of features (familiar advancement, bonus feats, etc) your Wizard levels and your PrC levels are seperate. For things that care about spellcasting (which is what Sublime Chord references) they stack.
How do you figure? Don't just read "As if you had gained a level"; read the entire sentence.
The text very clearly spells out what increases "as if you had gained a level." Your number of spells, your spells known, and your caster level go up as if you had gained a level. That's it.
Now, Echoes made this claim:
Your PrC levels stack with your base class levels for the purposes of spellcasting, which includes Sublime Chord's CL calculations.
That's just not supported by the text.
It doesn't say "You go up a level for any and all purposes involving spellcasting," which is what would be necessary for it to apply to the CL calculation of Sublime Chord (or Ur-Priest.) Neither one of those references your number of spells, your spells known, or your caster level.
Note that there are classes that do work this way--and no interpretation is necessary.
Compare the text of Ur-Priest:
To determine the caster level of an ur-priest, add the character's ur-priest levels to one-half of his levels in other spellcasting classes.
to the text of Apostle of Peace:
To determine the caster level of an apostle of peace, add the character's apostle levels to one-half of his caster levels in other spellcasting classes.
The ability's nearly identical, except for that ONE difference. Why would they use different wording if they wanted them to mean the same thing?
What's more, they created such classes BEFORE Sublime Chord (Apostle of Peace) and AFTER Sublime Chord (Knight of the Weave.)
Now, neither one is a particularly good ruling by which to operate, in my opinion; we use our own house rules for such classes.
But "Sublime Chord uses caster levels and therefore allows double-dipping" simply isn't supported by the text of the rules or by precedent. It's certainly a popular way to play, but it's not RAW.
For a long time, PrCs had spellcasting advancement listed as "Spells per Day/Spells Known". As of Complete Mage (maybe earlier) that section has been renamed to "Spellcasting". As to the Ur-Priest vs. Apostle of Peace argument, that's a strawman. No one is arguing that Sublime Chord counts CL alone (as that would lead to things like the Illumian's +CL sigil being counted, which I specifically mentioned as not being added together).
If you take Wizard 20, you increase your caster level, spells/day, and spells known. If you take a bunch of prestige classes that advance spellcasting, you increase your caster level, spells/day, and spells known. There is no functional difference as far as the mechanics are concerned. So tell me, where in the rules is there some mystical 4th quality to spellcasting that PrCs don't advance?
As far as I can tell, "spellcasting" is a term they came up with because writing "caster level, spells/day, and spells known." gets annoying real damn fast. Note that there are various abilities which provide only one of those. +CL boosts and +spells/day (Arcane Trickster RAW) are around. I don't think I've seen +spells known without +spells/day attached though.
