Amp is full serviced by a very good tech; caps, Sylvania 6L6GC biased 60%, known good preamp tubes thru played several known good cabinets at 4 and 8 ohms... This amp has the presence knob btw, what is going on is the amp has a 'slight bit' of 'static-crackling noise' at lower to medium volumes BUT after the amp has been on and played for say 20 or more minutes it seems to go away. A variety of guitars etc and the amp sounds very good at clean volumes once warm, so, could there be some bad resistors at work here until it gets warm? Using a compressor helps a bit as well BUT at medium slight gain tones it still gets a bit of a crackling sound. When you drive the amp very hard with various OD pedals it seems to go away somewhat. What else could it be? Its the 'edge of distortion' sounds that sound this way compared to say my 63 Tremolux which excels at 'edge of distortion'.....Change out the big bottle Sylvania 6L6GC for say RCA Blackplate 6L6GC for comparison? Once the amp is warm the clean tones are very good; presence knob allows lots of dialing in along with the treble knob for those unfamiliar with this 6G7A circuit. Any ideas? try some different preamp tubes (all are 12ax7)? Thanks in advance!