R' Chaim Kanievsky shlita davens Ashkenaz and wears Ashkenazi tefilin. His grandfather z'l was a Chernobyler chasid. It's not clear to me how the Steipler Gaon ztz'l conducted himself with regards to tefilin and nusach. The sefer Orchos Rabenu might shed some light (and if anybody knows where to buy this sefer, please share!). It is also known that the Steipler did not eat gebrokts, a chasidic minhag. When and why was the switch made from Chasidic to non-Chasidic customs, and what was the basis for doing so according to the one who made the change?
The Steipler kept his chassidish tefillin but bought Rav Chaim Ashkenaz Tefillin, although the steiplers father was a hornsteipler (chernoybl) chossid, the steipler learnt in navordok, a litvishe yehsiva and married THE litvish'e gadols sister (Chazon Ish's sister). I am pretty sure he continued to wear a gartel (but then again so did the chazon ish, it wasn't necessarily a chassidish thing.) I assume he davened nusach ashkenaz. Rav Shlomo Zalman Auerbach is similar in this regard to the kanevskys. VEKM"L...
This says that gebrochts was a private neder.
"He did not eat gebrochts but allowed his family to do so. Because of this, his meals were prepared in special keilim separate from the rest of the family’s keilim. In his old age, when it was hard for him to eat matzoh unless it was soaked in water, he was matir neder and ate gebrochts as well. (Toldos Yaakov)"