I recently aquired an old (1991?) SWR SM400. It has a slight noise problem. There is a low hum as well as a low crakling/hissing (I say crakling because it is not a constant hiss - up and down) coming through my Eden 210xlt speakers. I have done searches through previous posts but I have some unique variables. - Bass is not plugged in - Noise is just coming through speakers/horn, amp itself does not hum when speaker unplugged or plugged in - Noise does not increase when gain or master is turned up!! The noise is there when both are on 0 and never changes. - The crakling (not the hum) does go through the inbuilt di (only just, but it is the same type of noise as the crackling through the speakers) - I changed the tube and both noises where still there - not sure but they may have been lower. It was an old tube I had lieing around so it I cant guarantee it was perfect. I have ordered some more tubes which I am going to try I have an SWR350 which to compare it against and it is as quite as mouse so I am not just paranoid about the horn hiss. I dont want to pull the tube out of my 350 as I have sold it and don't want to mess around with it before the guy picks it up. I can probably live with the hum (rather not though) but don't like the crakling going through the di Is it just the fact it is old and I can't do anything about it? I actually had it shipped from the US and took it to local amp tech to change the power supply rating (we run on 240 volts). I later rang him after I got it back to ask about the hum/noise - he said he didnt notice it (yeah right)he said there was probably nothing I could do and this could be normal for older SWR's - does this sound right?? He is the best in the area although not experienced with SWR. Amps here in Australia are expensive and I've been waiting a long time to get this one so I want to do everything I can to get this one right!! Sorry for long post. Any thoughts??