Donald, when it comes to gain switches I think Young (socrates63) is referring to amplifiers that either through the use of series resistance on the output or resistors within the circuit manipulate the perceived gain of the amplifier. With the first example there are obvious downsides and pitfalls but lots of solid state designs are able to change the gain stage on the fly without adversely affecting performance. In my experience this is pretty difficult to do with tube amps short of adding a simple voltage divider (or similar) to the input section.
For commercial examples have a look at Headamp and Headroom, both of whom offer gain switches for most of their products.