Question on tube noises

After a couple of weeks with what is a completely new system I'm beginning to get past some of the awe it initially inspired and into some of the more esoteric "annoyances." The pre-amp is a Sonic Frontiers Line 1 with stock tubes. As expected there is some very minor tube hiss. I can only hear it by placing my ear close to the speaker when there is no input and it doesn't seem to be effected by volume. My understanding is this is normal. What is of concern is that in one channel the sound is contant and uninterupted. In the other is there is some minor crackling and popping. Is this normal or is that a sign of a bad tube? Comments on when others replace tubes would also be appreciated.