My Walkabout has been malfunctioning lately. The sound is almost inaudible, but there if I turn the volume all the way up, then it cuts in at full volume. Sometimes there is no problem at all, which makes it difficult to isolate what might be the problem. Could the problem be with the tubes? This is the first amp with tubes I've had (only in the preamp, for those unfamiliar with the Walkabout). I was under the impression that preamp tubes didn't really wear out, but I'm hoping this is the problem, as it would be an easier fix than getting the amp fixed by a tech. What else could it be? What can I look for to isolate the problem?