There are a couple of additional factors that need to be considered, which in some cases will be far more significant than your choice of a splitter. That is especially likely to be the case if either of the source components has a tube output stage (implying the likelihood of a high output impedance), and/or if ANY of the cable lengths are particularly long.
The parallel combination of the input impedances of the two amps, which will be significantly lower than the input impedance of either amp, has to still be much greater than the output impedances of the source components. If you let us know the specific makes and models of the source components and the amps it will probably be possible for us to look up the corresponding impedances, and provide calculations.
The signals seen by your new main amp will be affected by the capacitance of the cables to the amp in the kitchen as well as by the capacitance of the cables to that new main amp. If the output impedance of either source component is particularly high, and/or if the TOTAL of the lengths of the cables to BOTH destinations is long, upper treble rolloff of the signals to both amps may result, depending on the capacitance per unit length of the specific cables. Again, let us know the makes and models of the components, and also the lengths of all of the cables, and the makes and models of the cables if you already have them.
Regards,
-- Al
