@pg62 , REL does what most of the subwoofer manufacturers do. For reasons of cost and complexity REL designs it's subwoofer to be integrated in under the bass of the main speakers. There is no high pass filter at all on the main speakers. This is problematic for many reasons but the single most important reason that this is a bad way to go about it is that you are missing out on the tweak of a lifetime. Your speakers have coaxially mounted compression drivers which your 15" mid/bass drivers cross over to at a high 800 Hz. Low bass requires even a 15 " driver to make long excursions. This wastes power but more importantly adds very significant amounts of Doppler and IM distortion to everything else the woofer is carrying and in your case it is carrying a lot! If you get a proper two way crossover and cross to your subs somewhere around 100 Hz your system will improve so much that your wife will notice it. Seriously, women are very sensitive to distortion.
The best analog crossover is probably the JL Audio which is pricey but here is an excellent 4th order Linkwitz Riley cross over for $190
https://www.parts-express.com/ART-CX310-2-Way-3-Way-Crossover-245-880?gclid=CjwKCAiAyc2BBhAaEiwA44-w...you will also require one more set of interconnects.
There is one even less expensive way to to it and that is to use the crossover in the RELs for the subs and get the appropriate sized capacitor and put it in series with the input of your amplifier channels. In order to do this correctly you need to know the exact input impedance of your amplifier. The equation is Fc = 1/2piRC. Fc is cutoff frequency or 3 dB down point, R = resistance(impedance), C = capacitance in Farads.
Turning this around C = 1/ 2piRFc. Lets say we want a crossover at 100 hz and the input impedance of the amp is 20 Kohms.
C= 1/ 2pi20,000 x 100 = 1/12,566,400 = 0.00000008 Farad or 0.08 microfarad. That would be one of these.
https://www.digikey.com/en/products/detail/panasonic-electronic-components/ECW-HC3B803JA/7567997That will cost you $3.00 for two of them.
Putting them in the amplifier is no big deal if you know how to use a soldering iron. You identify the wire going to the positive post of the input jack. Heat it up and lightly tug on it and it should come right off. Connect one lead of the cap to that post and the wire you just took off to the other lead of the cap. Make sure nothing can short out before you close up. Done. It might take me 20 minutes to do an amp. I'd be happy to work through it with you if you want to give it a try. You can always return the amp to stock configuration. The electronic crossover would more than likely sound better because it will roll your speakers off faster 24dB/oct vs 6dB/oct for the input capacitor but I think the capacitor will make a decent improvement. When I first started with subwoofers back in 1978 we were all stuffing capacitors into our amps. The very first subwoofer crossover the Dahlquist LP 1 came with a package of 30 or so assorted capacitors that you could put either in the LP1 or in your amp, made no difference.
