I purchased a new Audio Research SP16 tube preamp in January 2008. The vacuum tubes have never been replaced & the unit have been used on a daily basis. I know that sonic quality will deteriorate with tube age. But rather than wait until the sonic quality becomes audibly degraded, would it be prudent to replace the tubes now (i.e. after 3 years)? Would there by any advantage of replacing the tubes with an aftermarket variety or would it be preferable to order OEM tubes from Audio Research? Thank you for your help.