I am sorry we have not being able to provide much feedback to your question. Probably there are not many Sphinx owners using 6-inch f/8 refractors on their mounts to begin with.
However, there is one CN member (sctchun) that meets the criteria: He uses a Lunt 152ED on a Vixen SXW. I can’t speak for him, but I can refer you to some of his posts here where you can also find a couple of pictures of his setup:
By chance, I just happen to own a Vixen Sphinx SXD2 mount, same as yours! I also have a 6-inch f/8 ED refractor, but I have never tried it on the Vixen. From my perspective, the scope it’s probably too big (think large moment arm) for the mount to use it for high magnification planetary observing -my main application for this telescope, yours may be different of course. But that’s only a conjecture on my part, not actual experience, so I would not like to deter you from your initial quest.
I don’t think using a lightweight 6-inch refractor (like the scope that you are asking for) on the SXD2 could damage the mount, because it is well below its rated capacity. For example, I don’t have any trouble using a C11 OTA (30 lbs) on the SXD2, and checking on my records, my ED152CF fully loaded for visual requires 7 lbs less counterweight than the C11 to balance on my Losmandy G11 mount.
If you decide to pursue your original goal, be advised that you would require to use the Vixen SX Half-Pillar (8” tall) as a minimum for comfortable seated observing and to avoid collisions with the tripod –plus setting safe goto limits on the Star Book Ten. I am using a 12” extension on the G11 mount with the ED152CF for the same reason.
I hope that someone else with a similar mount may see your post and offer some input.
Best wishes with your scope decision!