TSSUB18 used to drive two 12 in Altos but now only one is driven -

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it seems as if the other output is not working. What do I need? I have tried to switch the cables - and only one output seems to work. ---- I just realized I was in the Links - but they were going to powered speakers. I think I tried to connect to the regular outputs and did not get any sound. But I can't recall if only one or two were connected.

Any suggestions would be appreciated. I hope the sub is still viable.