Q:

Amplify shadows in scene

I would like to easily amplify all the shadows on the stage, but not increase the lamp's power. Is there a way to do it?

A:

If by "amplify" you mean make the shadows sharper, adjusting the emitter's size down will sharpen them.  
If you mean make them darker, one easy way is to change the "Look" of your scene's Color Management (in the Scene tab of the Properties window, under "Color Management") to a higher contrast look, such as "High Contrast" or "Very High Contrast"
Filmic base contrast:

Filmic high contrast:

For more control, in Cycles you can create a "shadow catcher" object (Properties window, Object tab, under "Cycles Settings", tick the box next to "Shadow Catcher").  Put it on its own layer, set up a separate render layer for it, then use the compositor to darken the shadow catcher render layer and overlay it onto the other layer.
Here is an example involving an emitter, a cube, a ground plane, and a duplicate of the ground plane which serves as the shadow catcher.

Emitter is on layer 20.
Cube is on layers 1 and 11.  Layer 11 will be a "mask" layer, so no shadow will appear on top of the cube in the final composite.
Ground plane is on layer 1.
Shadow catcher plane is on layer 2.  
The render layers are set up as follows:
 
Note that Film must be set to "transparent" in the Render tab of the Properties window.  If you need to render the environment, you can add an "Environment" pass to your object render layer and composite it in.
Here is one way to set up the compositor nodes:

You can use the "Bright/Contrast" node to further enhance the shadows, but simply combining the shadow layer with the other layer using "Alpha Over" darkens the shadows considerably.
Here are the output layers (object layer, shadow catcher layer, combined):

And, finally, here is the combined output if I use the "Bright/Contrast" node in the compositor to set Brightness to -2:

