Hi Tim,
I can't help with your question directly, but can mention what I did with my 1999 V70 TDI.
I would say that if you can afford to convert to a Nivomat set-up that some cars came with (softer springs with the very expensive self-levelling shock absorbers) or fit an aftermarket pump-up shock absorber kit, this may be the best option.
I did not do that - I fitted new gas shock absorbers which raised the rear just a little compared with the old ones (but that is not their purpose, of course), and at the same time fitted Grayston supplemental springs (their reference 39S09 or 39S09H) which go around the new shockers.
This raised the rear to a sensible level for normal use and made if firmer when loaded and towing.
Whatever you do, good luck and enjoy towing!
Ian
http://www.springassisters.co.uk/39S09%20INST.pdf