If you get a chance get your ears on some Amiron Homes. They remind me of the HD650 tonally with a little more highs (still very smooth), not quite as elevated mids but still very prevalent, more detail, and much better soundstage. Incidentally they sound great out of a Schiit Valhalla (original version) OTL tube amp. I know the Valhalla doesn't have the typical tube sound stock but they can be made to sound very warm and natural with the right tubes. Anyways I really like the Amirons (which have an impedance of 250 ohms btw) with the Valhalla. The Amirons are not your typical Beyer sound and are quite smooth and natural to the ears even on tube equipment.To be honest, I like the fast transients of Beyers out of solid-state and they seem to sound proper our of slid-state. Biggist gripe with Beyers is their tuning. Tonality is terrible. Sennheiser is a different story. The tubes work well. Particularly OTL with 300 ohm drivers. I wanted to point out that, it doesn't mean we should expect any high ohmage driver to work well with tubes.
So if Sennehiser 300 ohms is end-game or they are the headphone one is dedicated to. BHC makes much sense. If one has lots of headphones including planars, go solid-state.
I agree with your assessment that if you have a lot of different phones with a lot of different impedances and only want one amp a solid state or tube hybrid (my personal choice over pure solid state) is the way to go. If you have some high impedance phones (I would say 150 ohms and higher) than it may be worth checking out an OTL tube amp. They are magic with higher impedance phones.
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