What causes the amp to blow if an RCA splitter is used at the pre amp sending two left signals to one amp and two right signals to the other.To the extent that the characteristics of the signals that the two paralleled amp channels are "trying" to put out are not absolutely identical, each output will be forced to drive a "signal" corresponding to that difference into a load impedance equal to the output impedance of the other channel.
In the case of nearly all solid state amplifiers that output impedance will be near zero, i.e., a tiny fraction of one ohm. The result MIGHT be ok initially, depending on component tolerances and other design characteristics, but as components age and burn in the differences between the gains and other performance parameters of the two channels will change, at least a little. And if for any reason the output stage of one channel were to ever not receive a signal, such as because of some minor fault in upstream circuitry in the amp, the other channel would wind up driving a full amplitude signal into the near zero output impedance of the failed channel, which essentially amounts to driving a dead short.
In the case of tube amps, since their output impedances are typically in the area of one or two or several ohms it might be somewhat within reason to parallel the outputs of the two channels. And in fact I believe that is sometimes done. But consider this scenario: A small signal tube serving one channel of the amp fails, at some point in the future. You start playing music. One channel is trying to put many volts and watts into the speaker, while the other channel (with the dead tube) is trying to force the speaker's input to zero volts. The outputs of the two channels are, of course, connected directly together through the speaker cables. As in the case I described of a failure in upstream circuitry within a solid state amp, ouch!
The bottom line: Don't do it, as the others have said.
Regards,
-- Al