[Sideband]

You need to decide where the distortion originates. Touching the grid of the EABC80 (with the volume control turned up) should produce a good loud hum. Touching the control grid of the EL84 should produce a somewhat quieter hum. If you don't want to poke around the valve bases, touch the centre contact of the volume control. If there is little hum from the volume control (make sure it's turned to full or nearly) check the anode volts of the EABC80. This is normally fed via a high value resistor (typically 220K or thereabouts) and may have gone very high or open circuit. If so, this will cause low distorted (sometimes no) sound.