I own many headphones from Sennheiser, Focal, Hifiman. Some of them are quite more expensive (Focal Clear MG, Sennheiser HD800S…). I don’t know if buying from Drop is the best deal around, sometimes it is, often it is not. I bought a few units on Drop including HD6XX, HD58X, O2 SDAC amp. I also own many others including HD600, HD598, HD25, ATH-M50X, Momentum 3, Momentum 4, Focal Bathys, PXC550II, Amperior, and more.
I am simply going to affirm one thing: If I was forced to keep only one headphone, only one, then this would be it: the Sennheiser HD660S2. I’m not suggesting this is the best headphone, I’m saying it’s one of the few I could live with as the sole headphone I could keep. It sounds good with any music genre. It is compatible with most setups. Yes it needs a dedicated amp. No it is not the best headphone for travelling or for noise isolation, but I could live with this just one headphone and enjoy music. Regards.
If I was allowed a second pair for outside/travelling Bluetooth use, then I would choose either the Sennheiser Momentum 4 (or the Focal Bathys, depending on budget available). Regards,