Thanks, David, for the thoughtful reply. I kind of agree with this - the HD650 feels like a right of passage sort of headphone, one that allows you to have a reference for everything else that's out there.
My main headphones right now are Grado PS500, which are a lot more expensive than the HD650 ( street price, anyway), but I think the Senns would bring something the Grados don't, which is all day comfort.
Funny, too, that you mention the Momentum on-ear, as that's my current work phone, and it's what got me thinking again about the HD650. I used to have the 598s, too.
In my experience, tubes alter the sound more so than amps themselves. That's one reason I got the Lyr, so that I could play around with different tube types without breaking the bank. And, since the Senns seem to reveal the quality of upstream components, the 650 should respond well to different tubes.
To bring it back around, I wonder what tubes would be the best match for them?
Looks like you are on a similar path on this headphone journey, not saying its good or bad.
Lyr2 and HD-650: best tubes IMO would be Telefunken CCa's, very detailed, great dynamics, best tubes that I have, unfortunately they are also the most expensive, and seem to be going up more. More economical choice: Re-issue Electro Harmonix (nice bass and extended highs), 75 Reflectors (choice of many) or Vokshods
I also have a RS2e, I liked it a little better than the PS500e which I tried at the same time, the aluminum outer housing made them too bright for me but the bass was nicer.
As for the Momentum on-ears: one of the best gifts the other year, gave 7 of them for BD/X-mas presents, girls love the pink, green and red, got all from Amazon, used-like new, $54-59, for most of them its the best headphones they ever heard.