[Derivative]

Whether is worth paying the price for IE900, only you can decide.
For me, IE900 was clear better than IE600.
Better bass, better midrange - I found the (generally criticized) dip being ideal for metal, while the IE600 sounded too shouty here.
For the highs, again IE900 is the winner, more detailed, more refined and not being too energetic/fatigue, just at borderline sometimes.
But I would'nt have paid the price for IE900 if not discounted under 1000 usd.

I second this - have owned both and the IE900 is worth the extra money and is a far more interesting proposition than the 600.

I recently plugged mine into my Little Dot tube amp, worked really well as the amp brought a nice warmth and coherence to the IE900s polished technicalities. This versatility makes me like them even more.

 

[RIGATIO]

I listened to the ie900 for around 8h non stop and i really dig them. Its a great set.
I think the Softears Twilight is a better set overall but the ie900 have some special flavour because of their unusual tuning. Sennheiser managed to balance everything very skillfully so the ie900 dont irritate me.
Twilight sound more realistic and "correct", especially in vocals and guitars and extremely smooth in the treble area. The ie900 have subdued vocals, so there is lack of energy but my brain addopts very quickly and i dont have any serious problems with this area. The trebles on the ie900 are very mature, detailed, smooth and clean but on some tracks they can get bit gritty, where the Twilight will never ever have this problem.
The bass on the ie900 is just AMAZING. Extremely detailed and satisfying. Is a bit softer than on the Twilight. Softears can kick harder with more physicality where the ie900 kicks with bit less energy but can go slightly deeper in the sub bass area. Twilight has more midbass that kicks hard, very hard with a lot textures.
Soundstage is just a psychoacoustics but im getting an impression that the Twilight is grander due to their open back design, like there is a more natural feeling of air but without boosted air region. The ie900 are closed back so they need to use more tricks to imitate big staging. And they succeeded. The stage is great on the ie900 but bit less naturally open than on the Twilight. Well its hard to ignore physics.
Great great and very interesting set. Bravo Sennheiser!

 

[Tybot]

A Sennheiser marketing rep on Head-Fi won't be able to do a lot other than suggest contacting customer support.

If anyone has a genuine complaint about a Sennheiser product after pursuing all the other routes I'd suggest writing a strongly worded letter or email to one or all of these people.

As of January 1, 2023, the operational business of the Sennheiser Group will be managed by a team of six: The new Executive Management Board (EMB) includes Greg Beebe (Executive Vice President Professional Audio), Ron Holtdijk (Executive Vice President Business Communication), Ralf Oehl (CEO Georg Neumann GmbH), Steffen Heise (CFO), Dr. Andreas Fischer (COO) and Markus Redelstab (CTO). Peter Claussen and Thomas Weinzierl, who were previously COOs responsible for Professional Business and Supply Chain, will also remain on the EMB before their respective retirements in the course of the years 2023 and 2024. In the intervening period, they will gradually hand over their operational tasks and extensive knowledge to the new EMB. In the future, Co-CEOs Dr. Andreas and Daniel Sennheiser will primarily focus on corporate management and strategy development and devote more time to customers, while the other EMB members will manage the operational business. Yasmine Riechers and Mareike Oer complete the leadership of the Sennheiser Group as an extended management team, representing the Corporate Development Office and Brand & Corporate Communication.

It's a great suggestion and I and others appreciate the info. I must admit though, I am probably going to end up not getting it fixed (at least not domestically) and will probably just steer clear of the brand in the future unless it's on the used market for $ I'm willing to lose upon failure.

The thing that really stings is that I'm more than happy to pay for a repair. 30% chance it was faulty IEM MMCX, 30% it was a aftermarket cable roll, 30% it was my fault and 10% my wife did it as payback for buying too many IEMs.