If you want to make a formant filtered sound, you need a synth where you can setup 2 parallell filters - one with a bandpass, and one with a bandcut (this one is most important). Then you control the bandcut width and the bandpass cutoff with whatever you like, and envelope, LFO, automation etc. Experiment with the knob settings on these parameters as well as the resonance on both of them to achieve the right movement.
The main formant sounds comes from 2 resonances in the frequency spectrum moving toward eachother, at least with the typical YEAY-sound. Hence why the bandcut filter is most important as you'll have 2 resonances moving as you control the bandcut width. The bandpass is more about balancing the sound.
Massive can do this, and Sylenth should be able to do this as well, however I don't use Sylenth, but I believe it has parallell filters.
Otherwise you could just do this OTB, using 2 parallell mixer channels with different EQ settings, or if you have a plugin where you can do parallell filters.
Anyway, I remember I've once stumbled upon a cheat sheet with what frequency parts that needs to be attenuated for what formant sound, so google it and see if you can find it (if you don't, let me know and I'll send you it, as I have it written down inside my DAW).
However I wouldn't recommend using formant filtering to create growl sounds... because they mostly sound really, really horrible and amatuerish, at least if you're going for a huge bass sound. But for funky pads and leads and such, or noisy effects with a more spicy shape on it, this technique can be perfect.
As for FM synthesis, it's a very recommended and a very reliable synthesis technique for this, but it's not the technique that produces the cleanest and smoothest growl sounds (like the ones Skrillex has), but more rough and naughty ones. Depends on what you want.
Check out this tutorial on a very advanced way to create very smooth sounding formant sounds:
How to actually make your own Skrillex Growls -
PS. "being modulated in 2-3 places"... that's cute