How do I run a sub

The A3.2 also has a set of RCA preout connectors that you could use if your sub has Left & Right line level RCA inputs.

Or, as suggested above, use another set of speaker cables.

I have an A5 Int Amp and run my sub from the pre-outs - works just fine.

Just run a second set of speaker cables from the amp to the sub.

My current amp has only one set of outputs so I use banana plugs on the speaker cables and spades on the cable running to the sub. Works fine.