The trompete is a reed stop with a bright and piercing tones. It is often used in the last verse of long hymns
or for fanfares.
This is normally considered a synonymn for Trompete, however on some organs it is used in the swell division as a stop with a softer tone.
When one uses a mixture along with a Trumpet-like reed stop it adds a higher frequences and sort of 'crowns' the sound and makes it more regal sounding.
A tuba is a very loud stop with a very full sound, this is caused by the air going through the pipe going at a very high pressure and the resonator being narrow and then flaring out dramatically towards the end