I am thinking of buying something to replace my (tatty and) old HD580s - how do 600s compare?
Similar for me, mine are bedside with old Sonos controller and Schiit Magni 2 and only get use when I listen to radio at night sometimes, probably similar vintage to yours being late 90s and I paid £80 new. Proper box etc and don’t know where they were made and although I thought about selling them recently I realised as well as still being good they have sentimental value to me from association with my dad. Main headphone listening now is through Meze 109 Pro.I was so lucky with my pair. There was a thread here ages ago that alerted a few of us to some redundant stock after Zavi went bust and they were knocking out brand new pairs of HD600s in slightly shop-soiled boxes for £99 delivered! The presentation box inside was in perfect condition as were the cans. I don’t use them very often, but I’d hate to be without them. I consider them a real reference point. I have an old ‘Can Opener’ from the 1980s so hooking them up to the outputs of a power amp is easy enough which is useful for diagnostics etc. They really are superb headphones, I can’t imagine wanting/needing more.
How do the 660s compare? I see they are on offer at 275 at the moment
The 660S is perhaps the better choice if you're using an onboard headphone output with restricted voltage reserves. For the same volume setting on such devices the 660S plays around 2 to 3dB louder to my ears. It also sounds a bit clearer and punchier with more grip/control in the bass and crisper dynamics in the mids and highs. With a decent dedicated headphone amp, however, the 600 pulls ahead.Looking at the Sennheiser website, the 660s are 150 ohm so will be slightly easier to drive than the 600 or 650.
Does that bring anything to the party if they are to be used without a dedicated headphone amp?
Sennheiser Outlet currently has HD660S in stock at £275.
https://www.sennheiser-hearing.com/en-UK/p/hd-660-s-refurbished/cl80a2wm866rw0bw5fnay84gh/
They come and go, as does availablity for HD600. If you want some, you just have to be patient and check regularly.