If you have the volume of your processor up so that the mains are showing around 80dB or so on the frequency response screen, then all you need to do is keep the response curve level to the left and you'll end up with the sub adjusted to the correct level. With so many places to adjust the sub level (sub amp, processor, and SMS-1), it does get a bit confusing, but I simply set my sub's volume nob at around 1/2 and set the sub trim on my processor around 0dB (I had it at -4dB previously), then left it up to the SMS-1 volume control to handle the rest.