OK, so I acquired a used Aguilar DB 680 pre-amp and I would like to replace the tubes (since I don't know how old they are). Suggestions? Is there that much difference in the tone of different pre-amp tubes? Some favorties brands and how would you describe the tone. Looking for a clean of a tube tone as possible. Not looking for grinding distortion. Amp will be Stewart World 2.1 with Bergantino HT322 cab, Lakland 55-94 bass. Thank all ya toob heads in advance.