The Nistar of Chassidim is generally based on the Lurianic Kabbalah found in the Zohar. The Chasidei Ashkenaz possessed a mystical tradition that preceded the revelation of the Zohar. Whereas Lurianic Torah tends to deal with upper worlds, souls, and intellectual constructs surrounding them (Chabad is a great example of the latter), the kabbalah of Chasidei Ashkenaz, while containing musar, often pursued more concrete matters such as the workings of angels and planets all the while pursing the glory of HaShem, much as one finds in the German synagogues with all their dignity and order. (As explained to me by Rabbi Tzvi Abraham)
The Rabbeinu Elazar Hagadol, Harokeach, is a great expositor of Chasdei Ashkenaz mysticism. See R' Tzvi Abraham's article here.
You can keep minhag Ashkenaz, daven nusach Ashkenaz, and connect to a mystical tradition that coordinates fully with it. This isn't to say that one can't study chassidis or Zohar. But here's a different way of going about mysticism.
The nusach of Minhag Ashkenaz already contains mystical items such as number of words in brachos. I have a Minhag Ashkenaz siddur that actually shows the word count for each bracha.
Side note: If you daven Nusach Ashkenaz, and even Minhag Ostreich (aka Minhag Poland) to some extent, you are davening a nusach that is full of Chassidic influence, such as Barich Shimei, Ana B'Coach, extra Tehillim, extra korbonos, and extra kaddishes.
The טשכנוב Rebbe held that one should not switch to Nusach sefard. Also the Komarna Rebbe and the Rav of Buczacz held differently than other Chassidic rebbes on this matter according to Michael Friedman of KAYJ. According to the Treasuries of Ashkenaz blog, Belz and Sanz daven a basically Ashkenaz Amidah. Boston and Berdichav also have their own nusach which have been said to have Ashkenazi elements (needs to be confirmed).