Actually a doubler, or teleconverter increases the apparent telephoto effect by a factor equaling the power of the teleconverter. A 2x effectively doubles focal length of a lens, but will use up (absorb) 2 f-stops. A 1.5 power will increase the focal length by a factor of 1.5 and absorb 1 1/2 stops of light. Quality can suffer from a little to a whole hell of a lot, depending on the brand.
Extension tubes change the relationship between the optical center of focus of a lens and the film plane (position of the film) within the camera. This allows the lens to focus much closer to an abject, which in turn causes the object's image to appear larger on the film. It also steeply shortens the area of acceptable focus in front of the lens and also absorbs light. by a ratio of 1 f-stop per 25mm of extension. Ther is no loss of quality, but focus is highly critical, as the depoth of field can be in millimeters.
You may be confusing doublers with a dioptric lens, which is a simple magnifying glass, made of optical quality glass, magnifies the image before it reaches the lens. These are often called "close up filters", but that isn't correct as they ar enot filters, but lok and mount sort of like a filter. There is no loss of light, but image quality may be somewhat degraded.
Distinctive Images
