I acquired a used DB 359 and have been enjoying it for the last half a year or so. Recently, after it is thoroughly warmed up (2+ hours or so), I sometimes get a soft pop followed by a hiss that goes away after 10 seconds or so. This is an intermittent problem. Sunspots? Tubes that need replacement? To generalize the question: How do you know when you need to replace tubes? Only after they fail? Or are there 'soft' failure modes where they still function, but not properly?