MOTM

> Ahh, okay. I'd never thought of it that way before. 
> Probably I've worked 
> with inferior AC-coupled synthesizers in the past so 0 Hz 
> is basically 
> "non-existent" because it gets highpass filtered by the 
> coupling capacitors. 

Nothing bad about AC coupling.

BTW, such circuits have been used as Frequency Doublers for
guitars. AC coupling at the output of course. (AC coupling
at inputs, too.) 
Now the amplitude of the input signal is not constant with
a guitar, so the "DC" voltage at the output is not constant
either. Strictly speaking, it's no more DC then, but a slowly
changing AC. Part of this will go thru the AC coupling, so
you get side effects from "thumping" to "tube like" dynamical 
shift of bias point for the connected circuity - a bug or a 
feature, depending on the application. And the Signal
amplitude at 2*f is the x**2 of the input amplitude, so
the signal becomes more "percussive". There have even been
circuits with a crude compressor at one of the RM inputs to
avoid this ...)

With a fixed input amplitude and AC coupling at the output
you get just the desired frequency doubling. (At half the
input amplitude).

JH.