3 three channel system, anyone?

IIRC, Dynaco had this accommodated in their pre-amps at one point in time (many, many years ago). I could be wrong.

Meridian, I know my 861 has it and I am pretty sure some of their other models do as well.

My Fisher tube integrated had a center channel power output in the 60s. You can do this with ANY preamp. Get 3 high quality Y connectors, male to two female. Plug one into the right channel and use one of the female plugs to run your regular interconnect to the amp, do the same on the left channel. Then run the ICs from the other two plugs to the third Y connector and plug it into your center channel amp. It carries a combined L+R signal. You are simply doing externally what the other preamps do internally and will have no more signal degradation than they do, assuming high quality connectors.

Meridian uses and older system called Trifield to accomplish this, it used to come on Yamaha receivers back in the day too, it works amazingly well, simply making a center of the left and right would be silly in my mind but YMMV....