Well, first it probably would be better to stuff the ports of your main speakers. Go to a drug store or similar place and buy a pair of women's ankle-high nylon stockings. Then go to Home Depot or similar and buy a small amount of rolled up fiberglass insulation (you can find small packages). Cut a piece that is about as long as the port is in the short dimension, then roll it up until it seems a little bigger in diameter than the port. Cut off the rolled-up piece and, keeping it rolled up, put it in the stocking. Tie off the open end of the stocking. Now repeat for the second speaker. Then put your stuffers into the ports of your main speakers.
Now, find some music that has decent bass -- a jazz trio works well because you can hear the individual double bass being played and it will span the range of your main speakers and subs. Set the subs' cross overs at the lowest frequency setting, and the levels at zero. Now, start playing the music. Gradually increase the first sub's level control until you are just aware of the sub as a separate sound source, then back off just a bit. Do the same for the second sub. Make a note of the level settings . . . you'll want them to be the same on both subs. If one sub's level setting is higher than the other's, turn it down, but not all the way to the setting of the other sub. Then turn the other sub's level control up, to the same setting as you established for the first sub. Does the bass appear not to be coming from the sub, but somewhere between the main speakers? If so, you've got it. Now, listen to the quality of the bass being played. As the player plays lower notes on the bass, does the loudness appear to be the same as the higher notes? If the player is running up or down the scale, is there a note "in the middle" where the loudness seems to falter? If so, raise the cross over setting just a little, and listen again.
This sounds tedious (and it is) but if you work at it, eventually you will get the seamless bass that you want. In my long experience fooling around with subs, I have found it is hopeless to try and set up a sub based on the frequency response specs of the main speakers and whatever is indicated on the frequency control of the sub. You will just never get there. The problem is that the frequency control on the sub is likely inaccurate, the indicated response of the main speakers is also likely inaccurate, and every normal sized room (i.e. not an auditorium) has a fundamental resonance somewhere between 40 and 50 Hz with a peak of up to 6 dB. And, with a ported main speaker, there is a phase shift at the speaker's system resonance point where the sound from the port predominates (below the system resonance point). This sound is 180 degrees out of phase from the speaker's sound above the system resonance point, which comes from the driver. As long as you're running the speaker alone, this shift doesn't matter. But, when you bring a subwoofer into the picture, it does because this shift takes place right about where the subwoofer and the main speaker are working together. That's why I suggest you stuff the ports of the main speakers, to reduce this output. This reduces your main speakers' bass extension, but you don't care about that because you've got the subs!
