Just got the HD650 today (for some bizarre reason, the HD600 will be delivered tomorrow) - first impressions are quite good.
Did a very quick A/B with my speakers (Focal Aria 906) and I think I actually prefer the Sennheiser - i can definitely hear the bass more clearly than through the speakers and despite many people talking about the HD650 being veiled, I find them and as clear as the Focal. The latter might have just a smidgen more bite though.
Anyway - as a total HP noob - my question is: is it normal for £400 headphones to be more resolving and detailed than £1000+ speakers?