I'm not an engineer but I really can't understand why ther aren't more of these, particularly for subwoofers.
I would assume (and yes, i do know the four implied wods within) that, not using internal power amps, two router sized units, one transmitter and one receiver, perhaps with wall-warts would be all that's needed. Heck, maybe make them stereo units so they could be used or surround speakers as well.
Oh, make them able to accept and output line-level analog audio signals
I think there's a market out there waiting to be tapped.
If they already exist, please enlighten me/us.