You can look in my sig for a review of the Merlin, might help things. Mine is a prototype before they boosted the bass to make it stand out more from the Miracle sound signature. The 8C is probably closer to the final tuning that UM decided on when launching the Merlin, in terms of sheer bass impact. But the UM house sound has somewhat more forward upper mids, more energy and excitement while the 8C is smoother and more analog sounding if that makes sense. Merlin is a great IEM, and personally I love my prototype model which I think is what keeps me from wanting to buy the Miracle (because they are so similar). But it's just not really in the same league as the 8C.
I'm not sure what changes may have occurred with the UM products recently. All I have of theirs from this year is the PP6.