I love speculation/guesstimations 8-)
There is little change in the PF with load. I guess the PF is to a large extent an inherent property of the of the design of the PSU. Presumably the PF is _leading_ for SMPSs. The PF of the SPeakers is doubtless a X-former hence its poor PF.
You have only got to see the amount of fins and fan(ette)s in a newer sys box to realise where the energy is going.
I guess that although the circuits are physically smaller there are many more of them (does this work out as proprtional to the area of the silicon in the sys box?). Not that much more? Perhaps the difference is that the circuits are also switching much faster in the newer box.