Like any good room correction system, MultEQxt concentrates on correcting the low frequencies more than the rest of the frequency range.

A couple of advantages with the Audyssey: it correlates data from measurements at multiple locations, so you have a better chance of catching problems compared to measuring from only the sweet spot; it also measures in the time domain, looking not just for low frequencies that are louder but also for ones that linger in the room and have the longest decay times.

I would first try doing everything I could with the receiver's built-in room correction. If you're still not satisfied with the low frequency response in your room, then I would consider adding the SMS-1.