Help! I have a 1954 DuMont hi-fi that has suddenly experienced a volume drop. All tubes are lighted, so I don't think it's that, I even replaced the 6AV6 preamp tube. I get diminished audio when playing records on the machine - I have to turn the master audio knob way over to high, and the bass and treble knobs also seem powerless. When I try and use its auxiliary input to play stuff off my CD player, the audio just falls away to next to nothing. Machine had been having an audio hum the past few months.
Inside the box containing the bass/treble/volume/and on/off phono/auxiliary selector knobs, there are at least five large capacitors wired into the system, and there is one that is black and looks very waxy...how can I test the capacitors?

Scott,
I wouldn't bother testing them. It sounds like you actually use that vintage piece of equipment, so the best thing you could do for it at this stage in it's life is replace all the caps and electrolytics. That pretty much guarantees it will out last either of us.
Jim