olid state amp with a input impedance of 100k to be optimal.That makes the choices much more limitedThere are few, having the Lightspeed Attenutaor really makes me look out for anything over 47kohms which would be ok for him.
I believe 100kohm should be the standard, that way any tube or passive pre can drive it like it wasn’t there, also you would'nt see the big differences between preamps we see, especially if the pre is capacitor coupled output, as 10 or 20kohm input impedance can see the bass roll off too early, if the caps not huge.
Cheers George