I found a BFD guide and it talks about buying a mic and a mic pre-amp too in addition to a ~$100 SPL meter and the BFD. Seems a bit intimidating. Any thoughts on this approach vs. the auto-eq included in an AVR?
You can use the SPL meter as your mic. You only need the BFD if you intend to actually equalize your sub's response. I did do this with great success as you can see by looking at some of the graphs posted in the link in my signature, but to decide if you even need to do this, you can just use the SPL meter + the software to see what your response looks like now. This may be way more than you want or need to do. First just play with positioning in your room and see if you can find a spot that produces sound you are happy with in your main listening position. If you can't find anything you're happy with, you can consider the BFD/REW solution but plenty of people get by just fine without going to this level of effort.
Check this post
for a simple picture of what you need to hook up the SPL meter to your computer and use REW to run a FR curve.