The Rebbe urged his Chassidim to buy the finest Tefillin they could afford, written by a truly pious scribe.
(Chayei Moharan 511)
The Rebbe instructed his followers to don Tefillin of Rashi and Tefillin of Rabbenu Tam every day. Reb Noson expounds on this in Likkutei Halakhos.
(Si’ach Sarfey Kodesh IV, 194; Likkutei Halakhos, Tefillin 5:27-32; ibid. 6:16; also see Shulchan Arukh, Orach Chaim 34:1-2; Pri Eitz Chaim, Sha’ar ha-Tefillin, 9. The differences between the shittos of Rashi and Rabbenu Tam, as well as Shemusha Raba and Ra’avad, are presented with diagrams in Rabbi Avraham Yitzchok Sperling’s Ta’amei ha-Mitzvos, p. 19)