Unfortunately the baseboard on the XXL does not stay flat, it moves pretty constantly, even under load as you clamp workpieces.
I have bolted mine down to the torsion box it lives on using filler strips under the steel straps, bolts and large washers here in the center and under the T slots left and right
That keeps the whole machine rigid and square, cuts down on spoilboard vibration etc.
Other folks have been less extreme and got good results from laying the base of the machine on a rigid foam or other methods which stop sagging.
I went with bolting down because I wanted to deal with the upward part of the vibration motion and deflections when clamping workpieces as well as just the sagging.