@kevinzoe, your approach sounds good and rigorous.
I used the subwoofer crawl method. I placed the first Rythmik sub on the listening chair with controls centered and both Sound Labs playing. I then played bass heavy music (Ray Brown’s Soular Energy is good for this). I then crawled around the room listening for the best bass and marking spots with tape. I chose the most convenient location with the best bass, placing the first sub there. I then placed a calibrated mic at the listening position. Using a combination of REW impulses, pink noise, and frequency sweeps I adjusted the sub’s phase, volume, and low pass filter for the flattest measured bass response. Pink noise playing with an SPL of a frequency minima you can see while adjusting the subs is very handy for this once you get things close.
I repeated the crawl method with the second Rythmik sub. (The best locations turned out to be the same as the first sub). I placed the second sub, and dialed in the settings for it. It’s pretty cool to see the peaks and nulls reduce as the sub’s phase and volume are adjusted. The dual 10” DIY sub was placed in a convenient “good bass” location and slaved to the 2 Rythmiks summed to mono.
Over the subsequent weeks and months I tweaked the volume and low pass frequency a tad on each of the Rythmik subs as I played different music and heard things that didn’t sound quite right.
One thing I like about this approach is that it improves the bass not only at the listening position, but everywhere in the room.