So how do these multi channel amps with independant output control actually work? And are there any currently on the market?
Unless it is an older or more obscure brand name out there, it is pretty unlikely you will find something in a multi-channel amp with separate gain controls. I believe they did that in newer designs since any controls, theoretically will add noise to the signal and users would be controlling the levels from the AVR/Pre-Pro anyway. Also, unless the power amp has a lot of juice, controlling the volume from the amp itself and depending on the setting, could limit its full "head room" capability.
If your heart is set on the two center speakers, it has been discussed in other threads here about the use of pro stereo power amps such as the "Crown" and other brands which are quite reasonable in price with lots of power. You can get one dedicated to the center channel speakers and they generally have separate gain controls for each channel.
