It is a well-established pedagogical guideline that one should avoid negative advice whenever possible. As teachers, we tend to accept the premise that it is better to say "do this" than it is to say "don't do that." Nevertheless, this mode of thinking tends to go out the window when tuba players approach repertoire that predates the tuba, especially anything by J.S. Bach. When it comes to imitating another instrument, the hip thing for the teacher to say has traditionally been, "Don't sound like a tuba."
I feel that even when such imitation is part of the process, the ultimate goal is still to sound like a tuba, even (or perhaps especially) if that means playing the tuba in a way no one ever has. Throughout the 20th century, the contributions of the great player/pedagogues (Bell, Jacobs, Phillips, et al) that have become the mainstream of tuba pedagogy were in their times often most remarkable for challenging the assumption that the tuba player's abilities need not be on par with the rest of the orchestra simply because the parts did not demand it. What better way to shatter that barrier than to beat the violinists at their own game?
As with composing, the first steps have been imitative. This doesn't mean that they have not produced anything valuable; it does mean that the story isn't over. I've heard it said that the current level of tuba playing in general has grown leaps and bounds over what it was even a mere 20 years ago. In any case, it doesn't take a historian to hear that we've moved beyond the now-traditional dynamic where the greatest thing we can accomplish is to sound like something we are not.
When we play, it must be explicitly with the intent of sounding like a tuba. As Gene Pokorny says in the introduction to his orchestral excerpts CD, if you want to be professional tuba player and you don't love the sound of the tuba, you may want to reconsider your career choice. We will continue to imitate all kinds of things in the interest of musical growth, but we must not be afraid to form our own idiomatic statements, and consequently, an identity based on sounding like ourselves rather than like something else.