To get meaningful low frequency measurements I aimed my pair of LABsubs outside through my garage door. This prevents room reflections from interfering with the measurement. Fortunately my garage is the most forward part of my house so there are no reflecting walls nearby.
The garage door and a couple sheets of plywood formed a baffle wall around the speakers for proper horn loading. The garage door is sheet metal but it did not seem to vibrate too much. Again, this loads the horns similar to one in a corner or four on an open ground plane.
I took the measurements at night when it was as quiet as possible outside. I used a full range MLS signal at about 95 dB sound pressure level. Sounded kind of like distant thunder. Here is a photo of the test setup and the measured frequency response (the absolute SPL numbers do not mean anything):
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