Tube advice

I'm thinking about replacing the tubes in my x-10d (Musical Fidelity). Now I know nothing about tubes-I believe the type used is Philips 6922's-I got this from another news group.2 questions. 1.Does it have to be exactly the same serial number or is there a generic form which this number relates to and therefore others I can use? 2.Obviously I'm thinking about changing with more expensive tubes-I'd seen Telefunken mentioned-is it possible to make a major mistake with the more cash is better theory? I appreciate there may be a degree of subjectivity involved in the latter question. Any help appreciatted and thanks in advance, Ben