You`ll probably void the warranty using any tube other than what the manufacture recommends. If you`re not satisfied with the sound you have, it`s either you swipe the tubes or get another preamp. I think it is rare that any mishap will occur using any tube other than recommended. Just buy your tubes from a reputable dealer.
In my VTL pre, I am using the newer Gold Lions. I have several tubed pre`s, and I have swiped tubes without any issues.