My experience adding subwoofers to 2 channel

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. 

m-db,

     I'm sure you'll be thrilled to know that setting up a 4-sub distributed bass array system requires absolutely no bass room treatments, mics, DSP and room correction software or hardware.  Room correction is completely optional. 
      It's also scalable since you can use any 4 subs to achieve accurate, smooth and detailed bass with your choice of available bass power, dynamics and impact, from 4 small DIY subs to 4 JL Audio Fathom V2s and any bass quality and capacity levels in between.
     Since there's currently no recorded vinyl or disc source material with bass content below 20 Hz,  I'm using (4) two foot tall ported subs with one sq. ft.  footprints and 10" long-throw aluminum drivers that are flat down to 20 Hz at 113 dbs.  
     My DBA system provides accurate, detailed, smooth and natural bass that integrates seamlessly with my 6x2 ft. fast Magnepan panel speakers which are only capable of bass extension down to 35 Hz.  It works like a charm for music and ht.

Tim

firstnot:"Swarm + Room Correction and you are set. Cost about $7k new.
$3.5k used. $1.0 for the very patient smart shopper."


Correction:

Swarm + No Room Correction required and you are set.  Cost about $2,800 new, including 4 ported subs with port plugs and 1,000 watt class AB sub amp with controls for volume, phase and single band eq.
No used price known due to it being the last sub system you'll ever need or want.

Tim
 

vinny I think you figured it out- when it sounds like just the mains, that's what you want. Some recordings have great low bass but a lot have very little or none. This makes it real easy to set the subs too high and not realize it until you get the rare record with really good bass. The meter test CD thing can get you close but then I think it just takes a while of listening and tweaking. Duke said the same thing, expect to spend a while fine tuning the levels. Then you put on Welcome to the Machine, Seal, Bird on a Wire, something like that, totally worth it.