Keep in mind all of this is highly subjective, as we all know one person's ideal signature is another person's bloated bass cannon or bright and brittle presentation.
Full disclosure: I primarily listen to my 2-channel stereo setup at home in a fully treated listening room with Dirac room correction, so my personal preference is for accuracy, natural tonality, and clarity/detail retrieval. This results in something similar to a Harman frequency response. With that said, strong YMMV qualifier:
Purely in terms of sound quality, I thought the MTW were decent, but simply not as good as the Mavin Air-X which is selling for $120 right now, and has become my gold standard in TW IEMs. The Mavins are a warmish take on neutral with the best clarity and detail retrieval of any TW model I've tried. They're balanced and neutral throughout the frequency response with a tastefully elevated bass and an extended treble that's among the most natural-sounding I've heard from any IEM. The Senns sounded somewhat U-shaped in terms of frequency response which resulted in a sound that was too aggressive for my taste, and weren't as good as the Air-X in terms of imaging and separation. Even after trying to use the Sennheiser app's EQ feature (which isn't the best... give me a real banded EQ) they never sounded right and sounded cold to my ears. Solely based on sound quality, the Air-X are significantly better IMO. Beyond that, add in the Air-X's 10-hour battery life, a case that stores a ridiculous amount of charges (I charge it maybe once every few weeks), Qualcomm TrueWireless Stereo Plus, and bug-free operation out of the box, you're really only missing firmware updates and a dedicated app (which tbh I couldn't get working consistently with the MTW when I owned it).
I ended up returning the MTW because of the battery drain/charging situation, the flimsy hinge on the case, the app that worked infrequently for me (YMMV), the goofy EQ functionality, and the fact that even when working flawlessly the Sennheiser isn't even capable of battery life to last a cross-country flight, and I simply don't understand how they can be asking $300 considering all the competition on the market. I even preferred the sound signature of the Bose SoundSport Free, although the MTW was better in terms of connectivity reliability and passive noise isolation.
I have no problem shelling out for an expensive TW IEM if it's worth it, but I just found the Sennheiser had too many compromises, and I consider myself a fan of Sennheiser in general. I hope they improve their second gen product whenever they get around to making one. The MTW I very much see as a "TW 1.0" generation product, whereas the "2nd gen" TW models coming out now are a significant step forward in terms of sound quality, battery life, and ease of use.
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