I've seen this mentioned many times in home theater forums.Search for "sub crawl".
I think, a better idea would be to use a mic with a laptop and Room EQ Wizard. It's a free application that can be used to do acoustic measurements and so much more.
You can do measurements in real-time btw. It would be better to use a calibrated mic, but for the low end any mic will be better than winging it without a mic.
Here's a $20 calibrated mic by Dayton Audio:
I used it when I installed some DIY acoustic panels on my studio and home theater.
This is before the panels:
This is after the panels: