array of reasons:
tube(s) going bad -- swap tubes between channels one by one and see if problem moves to another channel.
cap(s) going bad -- visually inspect circuit for any out of shape caps(mostly DC): leaking, bubbling, bending.
poor cold solder joints -- visually inspect under loupe solder joints mainly around ground path, but would not hurt to inspect the entire unit, but start from RCA input plugs and speaker terminals first especially if you've been recently reconnecting components and wires frequently. Cold joints could look like ones cracked, rash-shaped, dull.
As you go over your unit, clean the dust inside the chassis.