I like the Combination of tube and solid state for biamping. To me it's the only way to do it. Not sure why people biamp entirely with solid state. Unless The amplifiers don't have that much power to begin with and you need to supplement things.
Separation of amplification for tubes to the upper range drivers makes a great deal of sense. Tubes are especially nice for upper range drivers which handle mostly vocals and the harmonic benefits tubes is really nice. And then the focused power from the solid state amps focused on lower bass drivers is very nice.
The combination of 601 solid state and 2301 is very nice. I say this because I have use the MC 901 for McIntosh which is basically this combination using a 611 and a 2301 join together with an internal crossover in one big giant box for each mono. Avoids all the external crossover device. Youll always have the crossover stuff going on inside the speakers unless you strip it out which is challenging to address.