Very good video though I would add three points of clarification.
1. when adding multiple subwoofer maximum gain is as follows, with 2 subs 6db, with 3 subs 9db and with 4 subs 12 db. In practice because of room issues the maximum gain is never reached unless the subs are co-located.
2, getting phase between the multiple subs and the speakers is trickier said than done. The best way to get it right is if you have the benefit of a visual display of frequency response, something which you get with REW or and SMS-1. Only then can you see the immediate impact of switching phase among the subs.
3. Equalization is a last resort after palcement and room treatments. However I suggest it is much more beneficial in a multiple sub setup than in a single sub setup which only optimizes the sweet spot. Some room nodes can only be cured with modest equalization. Problems occur from ringing when too much equalization is added or from reducing the efficiency of the amp by adding boosts.
