My VTF-15H replaced at CHT-12 too and my response was pretty much the same. I love this sub.
I would guess that the average is somewhere around 9-9:30 volume setting for the sub. Itís been a while since I set it, but I think that mine is closer to 8:30. The level of -8 in your Denon means that the volume on the 15H is a little high. Turn in down a little and rerun the auto setup. Ideally you want the sub level in your Denon somewhere between -3 and +3. Also, when you run the auto cal set the sub to Q=0.7, EQ1 and one port open. Afterward you can return it to EQ2, Q=0.5.