Can I mount a different speaker in the BlockDock 12?

The BlockDock speaker is front mounted, and the distance from front of baffle to inside rear panel is 152mm (6"). So long as your speaker has an evenly spaced hole pattern (4 or 8 holes) and it meets the 6 inch depth requirement it should work. Please do note that if it is exactly 6 inches, you may want to be sure your speaker does not have a rear vent which may be blocked by the rear panel.