There's no right answer. I play this on C, because that's what I have, but it's not so high as to be ridiculous. If you look at the score of the entire opera here, I think you'll see why many players would choose a contrabass tuba when performing Lohengrin. There's certainly no reason you couldn't perform it on a bass tuba, though.
One of the keys to this one (in an audition setting) is fitting into the horn/trombone sound even when playing alone. Too many players play the high octave just way too loud. You can let loose a bit more on the restatement in the lower octave. And, please, no vibrato...especially on the long high Ds...I hear this mistake a bunch.
Does anybody else "play along" with the horns on the first statement on up to high G?
Even if you don't, you should sing along in your head with that part...it makes coming in on that D a bit easier.
