If the spheres are totally gone, then its riding on tyres alone, this is where the bounce comes from!
I have been in many Xantias like this over the years, its horrible to ride in, not to mention dangerous. But its purely because there is no gass left, so the suspension will not be working at all. So its bouncing along on the tyres.
Watch your strut tops if it is at this stage, as flat spheres will put so much pressure on them, ive known Xantias to pop windscreens as they have had no suspension!
When ever i see a Xantia on the road (less and less often now) i always watch how they ride, 8 times out of ten they look harder than they should be, and 5 of those will be bouncing along on tyres with no gas left in the spheres. An MOT fail if nothing else.
Ive know very cheap spheres go pop straigt away as the system is pressured, so that could be the case with this, since it mentioned new rear spheres.
However if its not the spehres, it can only be height correctors, suspension set too high or a replacement of fluid is needed. Although that wouldnt cause too harsh a ride.
Oh, it looks like you have a bargain
Chris.