It really depends on what you want. They both have a sub-bass boost but the Merlin has more of it and is also a dynamic driver. The Miracle is much tighter but doesn't quite have that moving air subwoofer feeling. I own both the Miracle and the Merlin but unfortunately my Miracle is in China right now so I can't give you a true direct comparison. I can however give you a brief comparison between the two.
The Miracle is all about balance and it does it beautifully. Everything from the lows, mids, and highs feel like they are directly proportionate to each other. Also since the Miracle is boosted in the sub-bass, it gives you a sense of rumble and power that I haven't heard in any other iem or headphone. It's not a bass heavy iem and it's not overly impressive at first but as you listen to it, you begin to appreciate the subtle things that it does much better than all the universals out there. I've owned high end dynamic headphones (see my profile for a list) and I feel that the Miracles are technically better than all of them. I've heard a pair of stax 009's at a recent meet and I wouldn't necessarily say they were "better" than the Miracles either. I guess it comes down to your preference.
I just received the Merlin a few days ago so take these comments with a grain of salt as the dynamic driver will continue to get better as it burns in. Immediately I was struck with a type of bass reverberation that I haven't felt in iems for a long time. As far as dynamic drivers go, I used to own a pair of ie8's for a long time so I'm pretty familiar with their sound signature. The Merlin doesn't have any of that mid-bass bloat that the ie8 has. If I were to compare the two it is almost like a veil is lifted from the ie8 with the mids, and highs being much tighter since it's done by balanced armatures. When compared to the Miracle, I feel that it isn't quite as technical and I can see where the increase in price is justified. It is mostly due to the fact that balanced armatures usually give you a tighter sound and you can really tell the difference in the bass. It's all preference but the Miracle does sound like the more polished product. Also keep in mind that the Merlin does have a wider soundstage which I find is a plus to it. Isolation on the Merlin seems totally different from the demo version as it is completed acceptable now. You'll definitely notice a difference between the ie8 and the Merlin. I can say that wind noise still slightly affects the Merlin but not so much on the Miracle so if you're looking for "slightly" more isolation, the Miracle is where it's at.
*Bear with me on this one as it might get confusing but it's informative*
If you look at the frequency charts (which I can post later on), you can see that the Merlin compared to the Miracle has more bass, slightly less mids, and on my pair it appears they have similiar highs.. I do remember when looking at the older charts, the Merlins didn't quite have the same highs as the Miracles so maybe there was a revision of some sorts. Here is a breakdown of it:
The proto-merlin was basically like a Miracle with a slight sub bass boost. That was eventually scrapped as it was too close to the Miracle so they made the production model Merlin which had even more bass, less mids, and less highs. Now from what I see on my frequency chart, they went back to to the proto-merlin style but with more bass, slightly less mids, and similar highs.
I remember hearing the Merlin demo and thinking to myself that it was lacking a lot in the highs and the iem was all bass. It was so bad that I actually preferred my ie8 to it and wondered who the hell would spend $700 on it. Obviously the demo isn't quite what the true ciem is but it's nice to see that UM made a few tweaks on the design.
I need to get ready for work but if you have anymore questions feel free to ask them. I do think the Merlin is a worthwhile product to upgrade from a universal but it all depends on what you're looking for. I've always normally listened to bass heavy headphones / iems and still came to the conclusion that the Miracle was the best thing I've ever heard. The biggest question you have to ask yourself is how do you like the bass presented to you? Do you love a dynamic bass sound that encompasses you? Or do you like a tighter balanced armature bass? You really can't go wrong with either as the performance isn't that far off from each other but I do feel like the Miracles are overall a better iem.
Also here is a frequency chart to give you an idea. The Merlin is on the top and Miracle is on the bottom.
