Funny we have the exact same trio of headphones.

(Though my HD600 is re-cabled.)
I found that the K701 settled down a while back, maybe a couple weeks ago. Its sound did seem to shift around the first couple months.
I haven't been listening to the DT880 in recent weeks, so its status is still at about 300-400 hours. I don't know if further burn-in will affect anything, but I'll find out. It's time to burn those headphones in, now that I'm done burning in the K701.

Coincidentally enough the EC-01 is a tube amp upgrade I've been thinking about. Would you say it gives satisfying results with all three headphones? Is it versatile enough for the three?