Lewm, my plinth would qualify as a CLD heavy plinth (50lbs) a la Johnnantais so I am not sure there would be much benefit from anything much heavier, but a slate job would definitely look nice - it is something I might think about.
I believe if the Oracle is set up properly, which is not that difficult, it can be incredibly open, dynamic, with a big soundstage, and truthful in pitch. Some say it is a bit ligh or dry in the lowest bass registers - which is analagous to the same characteristics I found in my old Quad 63's. I put that down to accuracy or quality over quantity. Anyway, I cheat, because if I want more low end I just turn up the volume on the crossovers on my transmission line loaded woofers!
I also find the SP-10 is also very accurate in the low end, no bloat or overemphasis here.
One of the great virtues of the SP-10 in a heavy plinth is that it seems to be agnostic to different tonearms and makes every kind of arm perform as it should. I did not find any particular synergies or mismatching with various arms I have tried. My old Triplanar worked nicely as did a Fidelity Research FR64x. Based on the arms I put thru their paces I would settle on the Dynavector 507 Mk2 as being a really good match.