Two points:

First point: Here is a question on gluts that I pose to my macro
classes. Who Said This?

In the first place my attention is fixed by the inquiry, so important to
the present interests of society: What is the cause of the general glut
of all the markets in the world, to which merchandise is incessantly
carried to be sold at a loss? What is the reason that in the interior of
every state, notwithstanding a desire of action adapted to all the
developments of industry, there exists universally a difficulty of
finding lucrative employments? And when the cause of this chronic
disease is found, by what means is it to be remedied? On these questions
depend the tranquility and happiness of nations.  (answer below)

Second Point: While Keynes did use the concept of effective aggregate
demand, it was not as a concept related to price as it is in the current
textbook presentation. It was essentially used as a concept of effective
supply--suppliers made their production decisions on expected demand,
and thus aggregate supply became a function of expected aggregate
demand, which in turn was a function of aggregate supply. It was this
interrelationship which was important, and the demand terminology caused
much confusion over what Keynes's point was. Would that he had used the
concept "effective supply" rather than effective demand. (The first
paper I ever wrote in economics tried to make that point--it went
nowhere.) When there is a lack of effective supply, you can have an
unemployment equilibrium if there are fixed wages. That unemployment
equilibrium is preferred to the alternative "equilibrium" which is an
spiraling downward of the price level--undermining the asset market and
further hurting effective supply. That's the point Barkley and I tried
to make when we both independently pushed for a presentation of Keynes
with vertical aggregate supply and demand curves. 

The answer to the first question is, as Per Jonson pointed out in his
1995 Eastern Economic Journal article, Jean Baptiste Say. As usual,
issues are more complicated that they appear on the surface.

Dave Colander