With these subwoofers, there is more to the story than active vs passive. The subwoofers without built in amp like the Hsu TN cylinder and SVS CS cylinder are technically passive because they do not have a built-in amplifier. I prefer this approach of keeping the amplifier separate from the cylindrical unit, as the cylinder need not be modified in any way shape or form. However, Hsu's amp unit is a 100% custom job, with proper equalization and subsonic filtering and soft clipping included. The amps that SVS uses for their passive cylinder CS subs are more generic Samson amps that include none of these features and are not customized for the sub.
The built-in amps that SVS uses for their powered cylinder series are customized for their subs. However, I do not prefer this because the cylinder needs to be cut in order to house the amplifier. From an engineering standpoint, this is not ideal, though it is practical for some to have the amp built in. Note that this is not an issue with box subwoofers, as they have parallel and not curved surfaces where the amp can be attached with no integral harm to structure itself.
Please do keep in mind that these cylinders are huge, and much bigger in real life than you might think looking at a small picture on a manufacturer's website. Unless you can keep them out of sight, I would suggest going the box route.
