I should have mentioned, that's one of my only
true issues about the Phoenix (aside from the fit which can't be helped as it stands) is that they are not very open at all for open-backs. At least 50% I'd say. The Fidelios are much less closed.
If it helps at all, "open-back fun" is specifically what I bought the X2HR for (I've only had them a month) and I love them. Remarkably comfortable, plenty of bass extension, decent impact, very good (not amazing) detail, and very airy and open, and a ridiculous soundstage (sometimes unnaturally so, but never not "fun").