Did the 668B have sibilance issues?
I think the 669 is the closed version of the 668B. In the same way the 662 series and the 681 series are related. The physical appearance would imply such.
I've been listening to my 669 for sibilance. Sometimes there are spikes where the 's', 'z', and 'sh' sounds are louder but they don't drown out anything after. Some songs will not spike at all. Some will only generate a few spikes. And some will spike at every opportunity. I think it's just the headphone being sensitive and unforgiving of recording quality.
I do use the 669 outdoors. It has great isolation and the included detachable 1m cable has been very convenient. But its large cups make it a bit difficult for portable use. You can rest them on your neck but they reach up to your jaws and limit head movement.
There aren't many people who have both the 668B and 669, so comparisons are rare. But there are a few threads about the two, such as this one
. I don't have a 668B so I can't compare the two.