Thanks very much for kind words. After owning them for about a year I eventually moved them on. There were several factors relating to this and neither of them was dissatisfaction just habits.
First of all I didn't listen to headphones. Much prefer speakers and when made the jump to my current pair as soon as I first got the Densen and then Hegel the level of fidelity was stunning. Headphones couldn't compare but that's preference. Second reason was their open back nature. On a rare occasion when I listened to them in the late evening there would come a point my wife would come in a start complaining it's too loud. It's a drawback of listening room in the bedroom and wanting to listen way past midnight.
So these are very practical reason. I simply don't keep the component I don't use. And I no longer use headphones for Hifi. Just at the gym for non critical listening. But overall it's a very good headphone to have. If you like K550 these are a natural step up. Easy to drive however they prefer better amplification. One word of caution. I didn't think they paired particularly well with Chord Mojo. Combination sounded slightly bass rich and low mids congested.
Oh and one final though I'd look into a headphone that will have a similar sound signature to your system. And I don't just mean tonally but consider dynamic expression how vocals are presented, their musicality etc. Compared to Naim some would say AKG may be safe sounding so would be worth looking at other brands. Probably best to ask Dale at headphone section.