I argue that you should learn to use your ears a little.
Training yourself to hear the differences in SQ is immeasurable.
Put one sub at you LP, turn all the other channels off, and play a good track with prominent and consistent bass content.
The first time I did it, it took me an hour or so in an 11x15' room. I tried Pink Noise, Test Tones, Rhythmic Test Tones, and Music. Music worked best IMO.
I found that moving in and out of a corner and hearing how the Bass Quality changed was the key for me to help understand what I was listening for. Corners amplify Bass, but commonly can also lead to Boomy and Muddy qualities.
Once I heard and understood that change, it was easy for me to crawl out the room... maybe 15-20 minutes on my knees sticking my head in all sorts of places! (That's a pretty picture, ain't it!)
Regardless... Keep your ears and mind open to the possibilities, even if the location is a little outside of your "proper" listening area. Most importantly, you are looking for places where the Sub (in your LP) will sound crisp and clean in it's possible home. Identify 2 or 3 spots where you find good clean bass performance.
The best location, needs to be a home for one of the subs. You may need to fine tune placement by moving it an inch or so as you finish dialing in.
The second sub, try in one of the other two spots, take measurements before and after and see how it is.
Repeat to the extent you can or re willing, until you get the bass dialed in.
Idealy, you can see your subs away from the Towers, and get effectively 4 discrete LF sources in you area... this will go a long way toward evening out the Bass Response in your room.
Combining the Sub Crawl with Geddes' approach to multiple subs is my preference.
I have written before on the absolute necessity of multiple subwoofers for high-fidelity reproduction of modal region frequencies in a hom...
seriousaudioblog.blogspot.com
As the title states this is a recording of a talk I gave on the Geddes approach to subwoofer setup. It covers both theory and execution. While the content had direct input from Earl Geddes, the views expressed are my interpretation.
forums.audioholics.com
Now, it's time to get a little obsessive!