I am afraid I took the coward’s way out and used a set of test tones, at 5hz increments, and a sound level meter at the listening seat to adjust the sub until the response was linear. In my case the only real complication was that my original speakers were linear down to 40hz, which gave me the crossover point for the sub, but were only -5db down at 20hz. Since the sub had 40hz as its lowest crossover point I had to alter the venting on the speakers to get them to roll off more rapidly. I still have a +2db peak at 40hz but I can live with that. Since the original speakers had quite good deep bass the sub made little difference except for percussive bass, which had noticeably more bite, and that made the exercise worthwhile.
I am afraid I took the coward’s way out and used a set of test tones, at 5hz increments, and a sound level meter at the listening seat to adjust the sub until the response was linear. In my case the only real complication was that my original speakers were linear down to 40hz, which gave me the crossover point for the sub, but were only -5db down at 20hz. Since the sub had 40hz as its lowest crossover point I had to alter the venting on the speakers to get them to roll off more rapidly. I still have a +2db peak at 40hz but I can live with that. Since the original speakers had quite good deep bass the sub made little difference except for percussive bass, which had noticeably more bite, and that made the exercise worthwhile.