I think the obvious answer is that the SS amp designer would prefer that you also buy their SS preamp. Yes, SS amps usually have lower input impedances than their tube brethren, but SS preamps usually have much lower output impedances than their tube counterparts. The SS amp designer does not really want you to use a tube preamp with their amp.
Amplifier Input Impedance
Why are so many high-power solid state amps designed with such low input impedances. Doesn't that really low input impedance limit the range of pre amps that can be used? Are there technical reasons why designers make these impedances so low? Why not design your muscle amp with a really high input impedance so it will potentially work well with all pre amps?