The SMS stays in the chain as a low pass and PEQ for the subs. It also has a fixed 80Hz 6db/octave high pass built in. In your case, I seem to recall that the 121 already incorporates a high pass (or in this case "low cut") filter integrated into the speaker. If so, you could take a line out of your pre into the SMS and onto the woofer of your choice and a second line out of the pre into your main amps and on to the 121. In this set up, the SMS is out of the signal path to your 121s.
OTOH, the SMS also has a pass-thru. If you don't have a 2nd line out of the pre-amp, the SMS can output an unprocessed signal for this purpose. It will remain in the signal path of your mains, but they will see a full range line level input. Or you can route the signal from the pre through 80hz high pass in the SMS and your mains will see the 6db/octave roll off below 80hz.
BTW, the SMS will control up to 3 subs, but each will get the same signal. That is, room analaysis and PEQ are done with all subs set up in place, and the SMS will cumulatively analyze the room output. You then set EQ for flattest response (or for your taste) on that cumulative basis. The thing works amazingly well. The process involves a lot of trial and error (and a fair bit of time), but when you get a flattish line through the x-over region, you will be amazed at how good the thing sounds. At least, IME.
Good Luck
Marty
OTOH, the SMS also has a pass-thru. If you don't have a 2nd line out of the pre-amp, the SMS can output an unprocessed signal for this purpose. It will remain in the signal path of your mains, but they will see a full range line level input. Or you can route the signal from the pre through 80hz high pass in the SMS and your mains will see the 6db/octave roll off below 80hz.
BTW, the SMS will control up to 3 subs, but each will get the same signal. That is, room analaysis and PEQ are done with all subs set up in place, and the SMS will cumulatively analyze the room output. You then set EQ for flattest response (or for your taste) on that cumulative basis. The thing works amazingly well. The process involves a lot of trial and error (and a fair bit of time), but when you get a flattish line through the x-over region, you will be amazed at how good the thing sounds. At least, IME.
Good Luck
Marty