Another set of speaker cables (the spade end ones) was added for the high level inputs of our powered sub.
Works perfectly~
I do not own any subs without high level ins and outs.
Edit.
Just looked up that Polk sub. Do you have another sub with high level inputs? (speaker level) I have no idea how to hook up to line level using a Zen amp.
Someone will know here..  

I wire my REL similarly to Mike and Carroll, using the high level connections. I also need to use a jumper between the negative posts to alleviate hum from the sub, but it works and blends seamlessly. However, I believe your sub only has RCA connections, so you’d likely need a source or preamp with subwoofer out connections to hook this up in your system.

There are essentially three ways to connect your sub, based on the input connections offered on the sub. There is LFE, which is to connect to a HT receiver to send the audio signals from a movies low frequency audio channel to the sub. Then there is the low level/line in connection to connect with RCA cables to a source with sub out connections or to a preamp. Lastly, there is the high level connection which connects via speaker cables, directly to the speaker jacks on your amplifier. I find this to be the best way to connect the sub for music. Your sub has the first two connections, but not high level connections, so you'd have to have a preamp or source with RCA out to connect the sub.

I've never used or heard the Polk sub, to be able to comment or compare. However, based on the design of the sub, it seems to be primarily a home theater type sub. I've used both ported and sealed subs and I find the sealed subs are tighter and more musical than the ported ones, which tend to be more boomy and geared to HT. I have used several subs from different manufacturers, including Klipsch and the REL. The Klipsch reference sub I had was marketed for both music and HT, but was a large ported sub and never gave good clean bass for music, unlike the REL. It did take me some work to set up the REL correctly, with placement and adjusting the output level and crossover, but it now blends perfectly with my speakers to give a really great presentation.

I will say adding a sub is not without drawbacks. I find my speakers to be very fast, articulate, textured and clean. If I try to use the sub to accentuate the low to upper bass, the sound does get a little muddy compared to no sub. Perhaps some more work with placement/settings may help, but I don't imagine it could get as good as a great pair of speakers with excellent low end extension. I find my Omega's to have very good bass, but don't extend to to the lowest registers in music. For my use, I use the sub to fill in the lower bass that my speakers don't reproduce and though there may be better options to get the best sound, I am very very pleased with what I'm getting with my set up. The sub does blend very well and just disappears. I've considered trying a Z-Rock, to see if that may eliminate the need for the sub. I know it won't fill in that lowest region, but perhaps a boost in the low end may be satisfying enough and I'm sure it would be the cleanest sound of the two options, but I've not yet taken that leap to try. So for now I do find the REL to be a great addition to fill in the bottom end. They do one thing and do it well, so I can recommend it. I know others here use other subs like Rythmik direct servo subs, etc. Perhaps someone can chime in with more on an option like that, and what benefits they may bring instead.

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I have no experience with subwoofers, so this is very much FWIW/YMMV. But what you are talking about in electronic terms is connecting the sub in series vs. in parallel with the speakers. In that situation, and given the input impedance of the sub is (typically) very large the math says six-of-one half-dozen-of-the-other. To me, short speaker cables are always better so I'd go whichever way has the shortest cable run. But I'm curious if anyone has practical experience to compare with theory.