Currently I am using a Legacy Valve Head and 1x12 cabinet which connects to the 8ohm output of the head. If I decide to get another cab to run off of the same head, which output should it be connected to? The head has 4, 8, and 16 ohm outs.

What ohm cab should i buy to go with this setup? A 4 or 16 to simply plug into one of the unused outs?

Thanks, sorry if i'm not very clear, I'm a little confused about impedance/loads/etc.
If you provide me a link to the owners manual for that amp I could probably answer that question for you pretty quickly.
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