Do you know what if any synths provide good examples of each? I'm assuming the P12 would not be doing + only, and +/- also seems problematic if it's creating "upward bumps", but then if that means it's doing log/exp +/-, Dave would have needed to somehow introduce the log function for mod intensity?
It's a bit difficult to exactly tell what synth uses what type of linear FM, since that sort of information is never disclosed in any manual. In most cases, you'll only find wordings like "DX7-like FM," which is often a bit misleading and handles the whole feature more like an add-on gimmick. I believe the Jupiters 6 and 8 made use of + linear FM, while the majority of synths make use of +/- linear FM, especially ones with a VA engine like the Blofeld for example. The Access Virus on the other hand makes use of log +/- linear FM. Those are a few I know of through similar discussions or articles.
As for the Prophet 12 or Pro 2 (I can only speak as a Pro 2 owner): I would identify it as +/- linear FM as it clearly lacks the more clangorous outcomes that do occur with + linear FM, albeit I can't identify any inverted lower freqency modulations, no matter how much extra mod slots I use to increase the modulation depth. So either the whole applied modulation is shifted to the positive side, or one can't perceive it well enough because even at their lowest frequency settings the oscillators are still running very fast. It does, however, not seem to be a logarithmic version of linear FM because in some cases the applied modulation appears to be a bit weaker in the upper registers. The latter is no huge deal, though, as one can utilize the mod source Note Number as a corrective.
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