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?
Im no expert by any means. ...but i discovered my subs were canceling each other out....a simple db meter worked well for me....changing the phase on them cured that problem. .......the rest of it was hours of listening and placement. .....level adjustment. ..crossover adjustments. ....but thats the fun stuff. ....when you hear the subs over the mains it sounds like crap.....when it sounds like everything coming from the mains you hit it.....if thats any help? Some source material doesn't need a whole lot and some needs more........
Ive found if you get up real early in the morning and can get a few hours to yourself  (good luck) put on a good pot of darkroast coffee and calibrate your system. can  do your best thinking alone.... let your own ears be the judge. ..they will never steer your wrong...
You may be using too much of your svs subs . If your speakers are good to 29hz those 16 ultras will be running at a whisper and crossed as low as possible. Your room may not be able to handle the overlap of frequency . I run Cronus magnum ii into my 4430 horns and mc2300 into the 2235h woofers. I then use a bx63a to extract the lows to a pair of jbl 2245 18” in the 1983 design 12cuft enclosures (incredible beautiful response that makes your eyes well up ). Powered by two mc2300 amps bridged. The subs are tuned to 20hz . And barely have to move . It is all in the middle of the long wall of the room 56x36 x16h . Sometimes the bass feels like the room is a water bed . Silent low frequencies you can feel on your insides . And at moderately low listening levels . Anyways... start at an unnoticeably low level . Stop just after you can tell they exist. Also your placement may be too far into the corners or close to the wall . ..first post ever. 
I've just bought a couple of medium quality subs and after following Jim Smith's guide for proper subs set up, with an RTA on some steps, the 3D sound and bass energy Im getting from my system is incredible!
And I still have to keep on tunning them for better sound and then run Dirac Room Correction to get full perfomance.
Its incredible how the mids and the highs have improved with the subs.