I've done a couple of things in a similar situation, though with a different amp and sub.
One way to do it is to get a pair of these Y-adapters, plug them into the pre-outs on the amp, run a long interconnect to the sub from one of the female connections and a short interconnect from the other female connection to the main-in on the amp.
If you want to use the speaker-level connections to go to the sub, you don't need to worry about splitting the power between your main speakers and the sub. The sub will draw only a very small amount of power, then boost that signal with its own internal amp. That's the way it works on my HSU sub, at least. The speaker terminals on my integrated accept both spade and banana connectors so I simply attached the spade terminals on my main speaker cables as usual, then put banana plugs on an additional pair of small-gauge speaker cables, plugged those into the ends of the speaker terminals on the amp and ran the cable to bare-wire terminals on the sub.
One way to do it is to get a pair of these Y-adapters, plug them into the pre-outs on the amp, run a long interconnect to the sub from one of the female connections and a short interconnect from the other female connection to the main-in on the amp.
If you want to use the speaker-level connections to go to the sub, you don't need to worry about splitting the power between your main speakers and the sub. The sub will draw only a very small amount of power, then boost that signal with its own internal amp. That's the way it works on my HSU sub, at least. The speaker terminals on my integrated accept both spade and banana connectors so I simply attached the spade terminals on my main speaker cables as usual, then put banana plugs on an additional pair of small-gauge speaker cables, plugged those into the ends of the speaker terminals on the amp and ran the cable to bare-wire terminals on the sub.