I have been researching this for a few hours this morning and it seems like the deeper I dig the deeper the solution burrows. It looks like I may have to spend hundreds of dollars and on a mic, sound card, etc. Not to mention the time involved in learning REW.
My subwoofer will be in a carpeted basement. It will be an open media room with another room behind it, and I am not looking to achieve perfect sound. How do I pursue the "DB meter and some test tones will get you 90% of the way though." approach? Everything I research seems to lead back to REW.
Hence, "rabbit hole."
Frankly, you'll probably be very happy with your system and build.
You'll only become less happy once you start finding out that it can be "better."
Don't think of it as a chore, but as another aspect of an interesting hobby. If it doesn't interest you, then, by all means, don't bother.
If you want to bother, REW is an amazing tool, and AustinJerry's guide is a fabulous introduction to it. If you have a laptop with HDMI output, then you just need a USB mic. If you want to get a calibrated one, it will be a little more expensive. Considering your investment in equipment already, another $100 isn't really that big a deal.
You'll probably want a basic RS SPL meter, anyway. Their basic model is fine.
The manual way involves measuring the test tones with the meter and making the necessary changes to get the best response. It's very tedious, which is why everything points you back to REW. But you're welcome to do it the hard way:
Hint: use REW.