My experience adding subwoofers to 2 channel

My Kappa 9 speakers are rated to 29hz and they sound pretty good in my 18x24 room...powered by McIntosh mc1.25 amps...l was looking for another layer of bass to enhance the first experiment l took my SVS pb16 ultras from my theater room and tried them sounded terrible,didn't blend well..couldn't hear a difference until you turned in up then it rattled the room final experiment worked..l used 4 Velodyne minivee subwoofers(1000 watt rms class D sealed 8 in.) and after hours of calibration l hit it......lve got the bass response that exeeded my expectations. ....l should have done this along time ago....can anybody tell me of another subwoofer that may work even better?
Sadly, what I'm reading here is not unique.It's not the sub, it's the calibration and how little most consumers know about how to do it well.

Your best bet is a sub with room correction built in, or a pre/integrated with it like the new Anthem line.

I make my own speakers, and from my perspective, integrating a sub is practically crossover building.

Also, talk to GIK Acoustics. Great advice and great products. Their soffit traps and bass traps can really help.
It is like crossover building makes total sense. If l ran a test tone sweep through the mains with subs off the bass started falling off pretty steady below 50hz. Now it stays flat down to 25hz. It's the closest sound to a real drumset that ive had so far..l wasn't going for the deepest bass possible. ..l like alot of energy around the 50 to 30hz. range for my favorite music....The system didn't need a whole lot, but when the subs are perfectly blended with the mains it becomes a whole different beast.l should have done this years ago...l bought all four subwoofers used for 1000.00....l may consider some new Audiophile grade subwoofers down the road..such as Rel.or possibly some sealed SVS models....
Multiple subs is the way to go. The more the better. Once you get to where you are now though (4) then yes better quality subs will be better. Also having them spread around the room, basically one per wall but asymmetrically not just one in each corner, helps to get smoother flatter response. Where are yours now?