subwoofer gain knob

Originally Posted by JHAz 

Once you have things balanced so that test tones are equally loud for each speaker, most folks around here suggest making personal preference adjustments at the receiver. There's no real sonic difference, but the receiver makes the offset knowable and repeatable in a way that turning knobs on the back of the sub may not be. set the sub's volume control so that the resulting calibration to keep it equal with the other speakers is generally around the center of the adjustment range, especially if you want to tinker.
I don't know the MRXs in detail. AFAIK all receivers leave the low pass for the sub (from the crossover settings) in line whenever you divert sound from the non-subs, so any such sound would get rolled off. If you had an LFE channel signal with content up to 20 KHz, what happens depends on the receiver. Check whether the MRX has a low pass for the LFE channel. Many do. Some make it adjustable, some don't. Normally, there isn't "supposed" to be content above 120 Hz in the LFE channel (at least for movies).
Leave phase at 0 to start. Once you've calibrated, you could play content or test tones in the frequency range of your crossover, and see how the blend works. It's that "splice" that you need to get right. What the phase differences between the various speakers will be depends on the phase behavior of the speakers and their distances from the listening position so there's no "perfect" or even tyipcal for all. Many adjust the phase relationship using the receiver's distance setting for the sub. It accomplishes the same thing (delaying the signal through the sub to a greater or lesser degree until everything mateches up for max levels at the listening position at the crossover frequency). It is again more repeatable, and frankly simpler to make the incremental adjustments with a device that has a remote and a visible readout as to how far you've changed things.