How can I connect Polk Audio PSW 111 Sub to Active Monitors

I am planning to add a PSW 111 to my existing monitors(Behringer MS40), to check the bass level of my mixes. In my current setup, the output from Focusrite Saffire Pro 24 goes to the RCA IN of my active monitors. I see that PSW 111 has Line level RCA inputs, but the outputs are only Speaker level. I want to take the crossover output to my monitors.
 
Is there a way to do this?

The sub is powered, but there is no line-out. Looks like I can connect the output from SaffirePro to the Line-Ins of PSW111, but I cannot take crossover outputs for my main monitors from PSW111. I am trying to see if there is another way of achieving the same result..

The outputs from PSW111 are the wire outputs that go to passive speakers. I am not sure if those outputs can be connected to the Line-ins of my powered monitors

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How many outputs do you have on the interface?

Looking at the specs for the interface it looks like there are 8 outputs on it. If so..... connect one of the outputs to the sub. In the DSP control panel just set that output to either the right or left just like the ones now going to your existing near fields.  Use the RCA line inputs. Ignore everything else.

I have a focusrite saffire with 8 outputs.... 
2 go to my monitors
2 go to a stereo system for listening on the stereo speakers or headphones for tracking
1 goes to my Polk audio sub....

all the outputs are set up as stereo pairs all with the same set of signals.

The sub will automatically put out only the lower frequencies.