So... After an evening with the HD600 and HD660S's... My ears have chosen the HD600's.
I enjoy a natural and neutral sound with clarity and instrument definition and the HD600's deliver on all these fronts.
The HD660S, by comparison, was muffled and dampened, almost like being played from under a blanket compared to the HD600.
I have only tested with tracks that I know very well and from a variety of genres, pop, classic, folk, rock and trip hop using Spotify Premium, Tidal Premium and Qobuz HiFi.
I compared both headphones with the Oppo PM-3 and was surprised to hear more detail and clearer definition and separation with the HD600.
I cannot believe how good the HD600's sound (to my ears and with my set up) but I am very glad that I have found them.
I think the best surprise is that I often forget that I have headphones on and think that I have left the speakers on as the sound is so convincing and clear.
Thanks again for your advice.