+10 db = 10x power requirement.
+20 db = 100x power requirement.
Most AVRs are around 100w, so around +20db from the listed sensitivity of the speaker (at 1m).
If you sit more than 1m away you lose 3-6db every time your distance doubles.
1m = 0db
2m = -3db to -6db
4m = -6db to -12db
If you are worried about 6 ohms (and that's "nominal" or "minimal", not constant, anyway); then take the rated power from your source for 8ohm and 4ohm and presume the worst case.
What will a separate do? On paper that depends on the separate; though there are issues like the voltage limit on the rails which there's no standard reporting of but which is oft better on separates than on AVRs.