These are talked up quite a bit soundwise, and it's well deserved. Once they are seated well, they deliver clean sound with a whole lot of that thumping sub bass.
The sound is meticulously tuned to be as generalist as possible despite the big sub bass boost. My one slight issue is with the response around 8kHz, it comes in a bit too hot for some songs and skirts the edge of sibilance while also managing to sound a bit dulled off due to the rolled off upper treble. I would have liked a bit less mid treble and more upper treble (slightly less than the Grand Maestro would have been ideal) because these can be a bit troublesome to EQ.
First problem I have with these is the power requirements. My LG V50 is just barely able to power these on low gain mode, so I have to do aux cable tricks to get enough breathing room to apply EQ. This isn't an issue for DAP users who don't have to deal with this, but it's at just the right impedance and sensitivity to be annoying. Thankfully, there isn't much wrong with the MSE's stock tuning, so EQ isn't really necessary.
Second point is not a problem for me, but a problem I imagine many will have. The nozzles on this IEM are long, just enough to get close to where Etymotics like to seat. It requires a whole different set of tips from what you'd normally use to seat properly. I typically use Azla lights or Divinius velvets for regular IEMs, and those do not work well at all on the MSE. I settled on the Tanchjim APB-B tips for the narrower bore because of how deep these need to seat. If these are not seated deep, they will sound bloated and distorted, so seating shallow for comfort is not an option. For me it's not a problem because I apparently have larger than average ears, so be aware if you have problems with something like the Monarch Mk2.
Some annoying things about the MSE is the lack of a metal nozzle with wax guard and solid tip retention. The sound bores are wide open, which is not a good thing on a nozzle that wants to seat this deep. Ear wax will accumulate quickly and cause problems, so you have to get little mesh screens to keep gunk out if you want to avoid constantly cleaning the bores out. That, and the nozzle only has a gently sloping indentation for tip retention, which is not enough to retain some of the thinner stemmed tips out there.
Overall, these are worth going custom if the universals don't fit well. I lucked out because these fit like near customs for me.
I have these headphones for a few weeks. I have been using them on a daily basis when I'm away from home and feel they're a great substitute for my Final D8000 Pro, when I can't or don't feel like using them. I use them mostly while at the computer, either at work or at home. They do a great job at putting me "in the zone" quickly in my coding sessions.
I feel they require a fairly high power to be loud. At work I plug them to a Lotoo Paw 6000, through a 4.4mm at high gain, and set the volume at 45-50. At home I plug them to a Burson Conductor 3XP, through a 3.5mm at high gain, and set the volume at 20.
The sound of these IEMs is so beefy it's almost tangible. To be fair, I feel my other IEM, a InEar ProMissionX (10 BA drivers), is slightly more detailed, however I feel the FATMSE are certainly sufficiently technical to give me a realistic impression of my music library. For example, it happened a few times that I turned my back to hear if a person was talking to me, when actually it was a voice from the music. In other words, the FATMSE may be "fat", but to me they're not "fatty", if it makes any sense.
The bass punches like nothing else I've heard before. I would compare it to the Final D8000 Pro, mutatis mutandis.
I hope this review helps you get an idea of my experience with the FATfreq Maestro SE. This is my first review, I kindly ask you to upvote or downvote it: it will help me understand if I'm doing a good job. Thanks and good luck with your purchasing decision.