LinPlug Spectral - Manual (Page 102)

In this example we assume a Modulation Depth of 1.000 (maximum depth),
as you easily can see the Multiplication works as one would desire while
the Addition leads to a not as desirable modulation with longer times with
no signal (because the amplitude cant go below its minimum, which is
silence).
But why do we explain all this ? Because Multiplication also has its
disadvantages with certain modulations.
Lets assume you would use the Modulation Wheel to modulation the Main
Amplitude. Having it set to -9dB the Modulation Wheel (as a unipolar
source) would only raise it slightly and not allow you to control it from
Silence on. But thats exactly what most people want, have a look:
So, to sum it all up. Amplitude modulation in Spectral is always a
Multiplication, with certain advantages and the need to use a special
modulation source (Mod-Wheel Sub or Aftertouch Sub) to control it from
silence to the level set by the respective volume parameter.