Very hard without seeing the circuit to deduce what the issue is- if your getting sound when the pot is at zero sounds like you've wired the send/return in reverse (possibly) or you've got a short somewhere. re-check your connections- the send /return screened cables should have the screen wired to earth at ONE end only- usually with some heat shrink to keep things tidy. On my DC30 channel 1 is the EF86 channels send/return (mine was wired up by Nik) is yours the same? Have you tried replacing the EF86?
I rewired the send/returns and now have the ch. 2 volume working proper through it's range.... Still have some hum to chase down and am not getting the level of volume I should out of ch. 2 .... I am thinking the lack of volume might be a bad ef86 bought off ebay.. ??
I built this from a kit it's my second build so I have a lot to learn about electronics ...Also, when plugged into channel 2 does your ch. 1 volume knob when turned affect the volume level of ch. 2???