Small footprint subwoofer

I decided to go with the REL R218. This came with the high level cable. I have the option to high level or line level signal. What would be better to use on? For the high level, I am using an Adcom GFA-5500.

I like using the REL cable.   They recommend it and it works with most amps. If your Amp is fully balanced at the output , which i think it is , then I believe you do not want to connect the Black ground wire.  Only the Red and Yellow.  Float the ground Check with REL before going speaker level. 

I respectfully disagree you don’t need a sub that goes below 30Hz for music even in a smaller room. There’s actually a lot of

+100

I respectfully disagree you don’t need a sub that goes below 30Hz for music even in a smaller room. There’s actually a lot of info down there

Agree! Even with speakers with a stated low end of 35 Hz, I found the addition of two Rythmik FG12’s made a huge difference in sonic range as well as soundstage and overall presence of music...

highend64  I apologize for not being more helpful. While you moved the HSU slightly and it measured differently is not how to position a subwoofer.

The most effortless, quickest and practical way to find your rooms standing wave bass modes must be done with the -2dB HSU placed at your listening position because its the location you're actually trying to improve the subwoofers performance within the room. Think of it as a sort of reverse engineering. 

Connect one or two long inexpensive RCA interconnect cables (from Blue Jeans  or Monoprice cables) to the subwoofer.

Purchase or download a low frequency cadence tones that can be constantly repeated.

As the tones are being played slowly walk or crawl around your rooms boundaries listening for the strongest best sounding bass. Make a map of these locations within the room as you'll only have to do this once. 

With the HSU positioned now in the most convenient mode location readjust the subs parameters while listening from the listening position first with the bass tones then with music, by ear. There is no right or wrong as to what you feel sounds best. 

This method does not work with -6dB rated speakers since they simply do not play play low or loud enough to excite the rooms modes so there's nothing to integrate.

Room size has nothing to do with the delicate extra low bass. You can hear it in many headphones. So why give up on it with a -6dB sub bass speaker?