Depends on the output capability of the preamp and the input impedance/sensitivity of the power amps. No general numeric answer.
Sorry, Rodman99999. The voltage is not reduced with each split unless the preamp has inadequate current output. The voltage across every one of a fair number of paralleled power amps will be the same if (1) their input impedances are equal and several orders of magnitude larger than the output impedance of the preamp and (2) the preamp has more than enough current to feed those paralleled loads. Current/power will be shared but voltage should not.
09-25-12: Kr4Under any reasonable circumstances Kal's statement will also be true even if the input impedances of the amps are not equal. At audio frequencies, the voltage seen by any of the amps will differ from the voltage seen by the other amps only to the extent that the voltage losses resulting from the resistances and inductances of the interconnect cables differ. Those losses will essentially correspond to the ratio of the impedance presented by those cable parameters to the vastly higher amplifier input impedances, and will therefore be utterly negligible.
Putting aside possible effects at RF frequencies, such as RFI, at audio frequencies the inputs of all of the amps and the output of the preamp are essentially connected directly together, and therefore a given number of connected amps will all see the same voltage.
That voltage may be less than what would be present if only one amp were connected, but not to a significant degree if the criteria described by Kal, and by me in the other thread, are met.