It's not going to be consistent across a brand, either.
For instance, some Logitech models, the headphone connection is just a pass-through, where on others, it does run off the speaker's amp (and the volume is controlled by the speaker's controls).
It's not something you can figure out by looking at the specs, either.
Intel 2500K & Scythe Mugen2 | ASRock H67M-GE | 8GB DDR3 | Boot drive: OCZ Vertex 3
TV tuner: Aver HDDuet | Xonar DG | Fractal Design R4 | Rosewill Capstone 450M