The sub-base that comes with the 5625-20 is normally removed. If not, you will lose the height adjustment equal to that extra thickness. I test-mounted my 5625-20 to a General cast iron extension, and I would have been happy to leave the sub in place, because it seemed like it would clamp more firmly into the t-slot tracks underneath the table. But, when I needed to route a piece with a sequence of upward adjustments, I ran out of travel long before I completed the cut. Can't say I'm familiar yet with the method of clamping for the Grizzly, but I will soon know.
Oh, and by the way, I read a review of this router where a fellow pushed the quick release height adjustment without supporting the motor, and had it fall to the floor. Since my router is also new, I took heed.