I wondering how you got 7 cu.ft. internal from 20"W x 23"D x 39"L. My calculations show an internal volume of 8.6 cu.ft. According to John at A.E. the PR's displace next to nothing, allowing 1.6 cu.ft. for the sub and bracing seems a bit much.
Sure, they'll work fine, it's just overkill in most cases.Is it ok to use a 18" passive with a 15" driver or 12?
Yes.Will this rectangular set-up work similar to a more square set-up?
Yes, poly fill will lower the tuning a bit.I will be putting some 5-lbs of polyfill which may give me maybe 1-2db's more bass below 20hz or so, will it also lower the tune of the sub from the current 16hz to maybe 15hz or so?
15 hz for the subsonic filter will keep the sub at Xmax.Also, should I set the subsonic filter around the tuning point (15hz-16hz) or go maybe at 14hz-15hz?
Adding ploy fill will affect the tuning, you can get fairly close to finding the exact tuning frequency by using test tones. 5 pounds of poly fill will give the box an effective volume of 7.5 cu.ft. Modeling shows a tuning of 15.6 hz.Will I need to run individual tones in the 12hz-17hz or so area to find the exact tuning point of the sub?