I have used the HD650 with many Japanese amps (Denon, Pioneer, Onkyo) and also a Little Dot Mk3 tube headphone amp. But the ipod (it's a classic gen, with an internal SSD) wasn't improvable. Some pics and info
https://forum.tz-uk.com/showthread.p...=1#post5610350
I think the HD650s have been such a good purchase because they have lasted so long, and through so many upgrades of my other equipment. I am not sure that Bluetooth speakers, of any type, will age as well.
Back on topic. I am interested in the H1-equipped headphones because of the convenience (Bluetooth pairing/unpairing is a bind) and the potential for using them in my home theatre for Atmos support from my Apple TV.
But... the expense is a lot for something that I suspect may not last as long as the Sennheisers. And those still sound amazing.