Personally, now owning a Golf III and a 2000 Cabrio, it appears the Cab is a bastardization of the III and the IV. It definitely has the IIIs interior, sound system, mechanicals, suspension, et al., but has the IVs front sheet metal, lights, instrument lighting scheme, and trunk mechanism. That is, the license plate is in the bumper, and there is a handle in the hatch to open the trunk. However, the handle doesn't appear to do anything other than act as a lifting mechanism. That is, pulling up on the handle doesn't unlatch it, you have to do that elsewhere.
Anyway, the point is, the 2000 Cabrio, at least in the states, uses the Golf/Jetta III interior, and more to the point, that models stereo equipement. So, it looks like (when the tuner finally arrives) I will be replacing the head unit, cd changer, and installing a new amp. There is no amp currently beyond what the head unit supplies. However, apparently (reading the propoganda) the head unit has a sub-out which I do not believe is being used.
And you are right, the stok system sounds pretty bad, considering it's the top of the line as far as the Cabrio goes. I yearn for the day I can finally install our Mk2.