REL Carbon Special Subwoofers or the REL G1 Gibraltar Reference Subwoo

I am not a big REL fan. IMHO the way they suggest to set up subwoofers is defeatist. The best subwoofers today use two drivers running in phase at opposite sides of the enclosure. This cancels out any Newtonian forces that otherwise would cause the subwoofer to shake or vibrate. Turn up the volume on a bass heavy piece and put your hand on the enclosure. If you feel it shaking or vibrating that is distortion. Dual driver subwoofers are made by Kef, Martin Logan and Magico. There may be others I do not know about. I recently heard the KEF REF 8b and I thought they were excellent, not cheap either. 

Your speakers cross to the midrange at 500 Hz. This means your woofers carry a significant amount of midrange. Middle C is 254 Hz. Removing the bass below 100 Hz will definitely lower distortion noticeably in the midrange. In your case it will not improve headroom much if at all because the tweeter is the limit. You need to use a high pass filter of high order to achieve the best result. Most subs do not come with a high pass filter and the low pass filters are slow so the subs creep into the midrange which produces awful results. This is why low crossover frequencies are recommended. Digital crossovers however can go up to 10th order. As long as you use a stereo set of subs and a fast digital crossover you can easily run subs up to 125 Hz without damaging the midrange. This results in the lowest distortion. If you are worried about digitizing the signal IMHO the benefits outweigh the deficits. 

I previously owned Martin Logan subwoofers. To each their own, but I prefer REL over Martin Logan. I'm a firm believer in there are no absolutes in audio, only preferences. If possible, the only way to know what works best in one's system is through a home audition. 

@mijostyn 

This is interesting.  

Your speakers cross to the midrange at 500 Hz. This means your woofers carry a significant amount of midrange. Middle C is 254 Hz. Removing the bass below 100 Hz will definitely lower distortion noticeably in the midrange. 

My speakers cross over at 270.  I can't change that on my speaker (MUCH lower price).  Much of the vocal range is under 500 Hz.  I did not consider that it would be reproduced with a woofer.  

https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119164746.app2

@ricred1, I am only referring to two subwoofers in the ML line, the Balanced Force series. The others are no better than anyone else. There are absolutes is audio. In this case a subwoofer with two opposing drivers is going to perform better than a single driver sub of the same overall quality. Personally, I will not have any of them. I overbuild by own.

@12many , Yes! Isn't that something. Most woofers are actually lower midrange drivers. They are in a portion of the frequency range where we are very sensitive to pitch and distortion. Doppler distortion or Doppler pitch changes occur with objects in motion. The lower a woofer goes the further it has to move to project that frequency at similar volume. The motion becomes visible below about 100 Hz at a moderate volume. The smaller the woofer the more obvious it becomes unless it is used in multiples. The cone is travelling towards you then away from you, changing directions. The same thing occurs when a car passes you. If the driver leans on the horn while it is passing you the tone of the horn changes rather dramatically. Most of us have experienced this first hand. If not I am sure there are YouTubes that demonstrate this. In the case of the woofer, when it changes direction it is changing the pitch of every other note it carries. The larger the portion of the frequency range the woofer carries the more damage it does. In your case you are better off than the OP whose woofer covers another entire octave. Other than making more low end this is the other major reason to use a subwoofer. People will mention how much a subwoofer improves frequencies above the subwoofer range and this is why. What kills me is that most subwoofers do not include a high pass filter for the main speakers totally ignoring a major reason to use a subwoofer in the first place. 

Please, if you are going to use subwoofer use a complete two way crossover. If you really want to take it to the next level use a digital crossover with bass management and room control which requires a measurement microphone usually supplied with the crossover. Some of the very expensive units do not supply a microphone. They expect the dealer to install and adjust the crossover. They will sell you the microphone and computer program as an option. This is a very powerful system. If you think you can do it by ear, have fun trying. I tried for 20 years and could never get it to my satisfaction until I got a TacT 2.2X. It did it in about 1 hour and the bulk of that time was me trying to figure out how it worked. The manual sucked.  

Not going to argue about audio. I owned a Martin Logan subwoofer from their balanced force series. I prefer my current subwoofer.