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?
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. 
One suggestion, read Jim Smith's articles about adding subs:
Swarm + Room Correction and you are set. Cost about $7k new.
$3.5k used. $1.0 for the very patient smart shopper. 
In fact, you dont need a pro RTA, I did it with an Android app called Sound Analyzer App wich is enough for the job. 
You have to find the flatest bass response possible and doing it by ear is not an easy job. 
The app helped me A LOT to find the best location for the subs (among the ones I've had due to decor), the best direction, the correct phase and a good crossover-volume level adjustment.