I agree with hilde45 and audiorusty.
At -6dB even if you purchased five more of any REL "Sub-Bass" systems they all roll off the lower frequencies so dramatically so as not to excite most rooms bass modes. While this allows for room placement flexibility its at the expense of the subtle sub sonic extra low frequencies found in most recordings as well as broadcast and film.
Despite the speaker manufacturers rated low frequency your description suggests they’re positioned in the rooms null. No amount of equalization or optimization can overcome a null along with the huge amount of watts that would be needed to meet their rated output.
Return or sell your -6dB Sub-Bass speaker and purchase one or two actual -3dB subwoofers. It's very important to locate your rooms standing wave bass modes to position the subs in. Using the crawl (or walk) test while playing a low frequency test cadence, long (Blue Jean, Monoprice) interconnects and adjust to your taste.