Ken,
I tow with a V70 D5 and have no problems on wet grass, or indeed the snow we had on site last Easter. On one site it was wet and muddy and the owner was offering tow offs with his land rover. Declined the offer and had no problems with my then front wheel drive Volvo 850 T5. It would be nice to have a full blown 4x4 for towing, it would be a better tow vehicle, but in the real world only a small amount of time is spent towing, and my D5 gives 50mpg solo, 30 with the van on the back. I'm sure the S80 will be the same.
Gary