Why want to swap the optical tube? A 6" f/5 reflector is a fine and versatile telescope.
If having issues with ergonomics, and are not especially fond of widefield observations, I would rather replace your reflector with a C6 SCT (or even a C8, especially if your mount is equipped with the clock drive).
As for refractors, the 100/1000 achromat is a rather inexpensive but nice option, even if I fail to see it as an improvement over your reflector (less resolution, narrower fov, and still troublesome ergonomics); owning already a fast Newtonian I do not see the point for fast achros.
Among "apos", those around f/7 could provide you with the same fov as your Newtonian, even if at the expense of a noticeably smaller exit pupil, so probably will need to replace some eyepieces as well; depending on the relative optical prowess, these may, or may not, outperform your reflector in some circumstances (especially low-contrast features observed at high power).
If like widefield observations the NP101 (or one of its previous incarnations) could make more sense than most other alternatives