Thank you, Joebobbilly! The truth of the matter is that I like both sets of cans. I am a mainly classical listener, so the extra bass and timbral richness (and truth-telling) of the HD650s is already delighting me. The new cable was a big shock, as it sounded completely different to the stock cable. It took away the 'veil' which is so often referred to, but opened up the soundstage so much that there is a gap in the middle in some recordings. It also, briefly, imitated the digital glare of the Q701s on the upper-mids. That has now settled down. These are now my standard headphones. The warmth is now more subtle.
The Q701s are still splendid at some things. I am still trying to kill the digital glare by long-term burning-in. All my desk equipment is new, however. Audiolab M-DAC, Lovely Cube amp, HD650s. It's all burning in together. I think that the amp is a better fit for the Senns than for the AKG.
On the menu at the moment: The 1958 'Sacre du Printemps' with Lenny and the NYPO. A long-time recording for hi-fi show-offs, it still sounds fine, but no dynamic lower than mezzo-forte is favoured! Not subtle, maybe, but good for burning-in!
Interestingly, the 'gap' seems to be decreasing. Or my ears are compensating.