A few weeks ago I was going thru the same process as you are right now. I saw white LED's and thought "that ain't gonna work".
The X uses just one LED throughout the panel. A Cree XTE, times 72 of them. The three different settings just bump some of the lights on, then more, then all of them. The fans are programmed to ramp up along with the lights. At 65W one fan runs slowly. At 130W both fans spin, then at 190W both are spinning faster. I think that's what I saw anyway. Something like that.
The SGS uses two different Cree LED's. I'm not absolutely sure about this, but I believe the two settings on the S turn on just the reds, or the reds and whites both. If that's correct, the way the X switches work is completely different from the S.
For the X, A-51 recommends 24" X 36", and claims a max of 36" X 36". Going by that guideline you'd be pushing it. But maybe you could fill in partway thru the grow with a handful of Cree A19's or something similar?
A51's XGS spectrum chart is
here.
The SGS is
here. Notice there are two different charts - one for the red, one for the "white". While you're there, it's interesting to take a gander at the spiky charts caused by "traditional" grow lights.