In the early ’90’s, I tried like heck to integrate a sub into my two-channel music system. All the subs I dragged home just didn’t integrate well since they were all essentially home theater subs that were non powered with passive internal low pass crossovers. It just didn’t work and I gave up. All of the cabinets were either band pass cabinets with just a port exiting the box or a typical down firing, ported cabinet. There seemed to be no such thing as an audiophile subwoofer back in the early ’90’s other than maybe some JBL models which were not obtainable to a broke kid right out of college.
Fast forward to 2020 when I started planning a completely new 2 channel system and with nearly 30 years of more experience (especially in the pro audio area), integration of a pair of 18’s was very easy. All passive speakers with outboard electronic x-over made it easy. Also, a much better budget certainly helps!
My 2nd system in another room is also using a pair of 12" low frequency drivers in sealed cabinets supplementing 8 inch two way stand mount speakers. Again, passive sub cabinets crossed over by an outboard Sumo crossover into dedicated subwoofer amplifier and dedicated mid-hi amp. It’s easy when you’ve done it a few times and know about how things should be initially set up (crossover point & slope, gain structure, etc.). Tweek a few things with the ears and one is good to go.