Interesting thought, but fraught with problems. For starters, regardless of any advertisements to the contrary, a Pilot 240 is good for 10 watts per channel with both channels operating -- maybe 12 or so each when driven individually. In any event, count each complete amplifier as a 20 watt device rather than 30.
Beyond that however, understand that the center channel output as designed is a "difference" signal, not a summing signal. Therefore, if you adjusted each amplifier with a 240 block to produce the same output level from the same applied signal, there would be no output at the center channel connections. This is as it should be of course because if a true mono signal were applied to both channels of a 240 amplifier operating in conventional stereo mode with a center channel speaker connected, the center channel would produce no output in that scenario. This is because there is no "hole" in the center to fill with a mono signal, which is what the intention of the center channel output was for.
With a stereo signal however, there is a difference signal that can be developed between the channels, so there is an output that appears at the center channel connections in that scenario.
There are two ways that you could do this:
1. Devise a phase inverter to allow a mono signal to drive the two amps within a 240 chassis to be driven out of phase. In this fashion, the center channel output would then develop a summing signal, which is what you would want. The load impedance presented to the center channel output would need to be 8 ohms, where upon about 21 watts RMS would be developed at the output of that "channel".
2. Parallel the output of the two amplifiers within a 240 chassis. This would then yield 2, 4, and 8 ohm output connections, where again, about 21 watts RMS would be developed. This approach (as is the first one really) has many potential problems in that you need the tone, volume, and balance controls to be electrically set exactly the same, so that each channel is carrying exactly one half the load of each channel. That is usually a virtual impossibility to accomplish, meaning that you would need to strap the inputs of the two power amplifiers proper together within each 240 unit, so they would each generate exactly half the power of that channel. In reality, this second approach works after a fashion, but is still a distant second to using an amplifier that can actually generate 20 watts by design.
I hope this helps!
Dave
