I have the 1266 TC and Diana Phi. I'd say the Diana is close, and how close it is depends on your priorities. Tonality is nearly the same between them. Transient resolve and detail retrieval is slightly, but noticeably, better on the big 1266. Impact and physicality is suprisingly similar. Soundstaging is the big difference, with the 1266 having way bigger size of soundstage and noticeably better placement. I'd only get the Diana as a portable companion to the 1266, but your preference could vary if you really hate the 1266's unique fit.
Thanks for that info. It's the big soundstage that I love so maybe the 1266 would be the better option for me, even if I'm sacrificing some comfort not going for the Diana.