solution for "popping sound" from tube amp?

Hi! I have a Sonic Frontiers SFL-1 tube preamp. It has worked perfectly for the last couple of years (time I've owned it), now suddenly tonight there was a "popping" sound coming from my speakers that I traced to the pre. What is the probably cause, and how might I fix it? I understand this pre only has one tube, and I could probably replace it if required, if a tube going bad could be the root of the problem. Where should I buy a replacement tube, and is there a particular type/brand that you recommend? Any help for this tube electronics novice would be much appreciated! Regards, Ben