Hello David,
If the manufacture specifies a max towed mass for a model then the vehicle will be capable of towing it. But whether it will be a safe tow depends on many other factors as well as weight.
The source I use shows very different results for the Volvo C70 Convert. so I feel it would be unwise to offer specific guidance on your model
Because the chassis or body of a convertible has to be strengthened in different ways. it will affect the towing capacity.
Always check with vehicles handbook, VIN plate, dealer or the manufacture to see what if anything your particular choice is allowed to tow.