If I'm playing bass guitar, I say that I'm playing bass guitar, bass or the Watt-fiddle.
When I play bass, I say that I'm playing bass, double bass, contrebass, contrabass, bass viol, standup bass, upright bass, dog house or whateverelse I feel like calling it. I never call it bass violin. I'm one of those guys.
When I play tuba, I call it tuba, BBb tuba, BBb bass, or brass bass.
When I play Sousaphone, I call it Sousaphone.
When I play organ, I call it "foot bass."
When I saw Jr. high girls play the little two octave Rhodes keyboard back when I was in Jr. high, I called it "Bass Hand."
When I play mandolin I call it "Ultra-High Backwards Bass."
I hope this clears up everything for an English as a Learning Language (which is the most politically correct way of saying that).