The chassis I assume is the outer covering of the valve tubes. The other components (at the base) are usually covered by default. Usually this is removable or optional for tube amps mainly for vanity’s sake, though sometimes just to keep the valves exposed to flowing air.

The ‘pre-amp’ is the pre-amplifier, which as the name suggests, does all processing necessary before the amplifier gets the input signal and thus contains signal adjustment knobs or buttons. These usually include volume and gain but may have other adjustments for bass, treble as well as other stuff. Consumer oriented ones may even have microphone inputs for karaoke.

Amps usually have nothing but a standard power button.

Hope this is helpful…