even if you will use the gain control, you will sacrifice the dynamic performance of your system.
there are only 2 right ways:
1 is to remove built-in crossover and replace it by active between your amps. in this case you will sacrifice one of the main signatures of your speaker tha had been design to have a certain crossover at certain cut-off frequencies.
2nd is to use an identical amp for top and bottom(no crossover is necessary)
other ways are just for another toy purpose seriously.
The best result of achieving mix of tube and solid state is to have a tube preamp to your SIM.
the only good way i see your objective is to have another type of toy.
there are only 2 right ways:
1 is to remove built-in crossover and replace it by active between your amps. in this case you will sacrifice one of the main signatures of your speaker tha had been design to have a certain crossover at certain cut-off frequencies.
2nd is to use an identical amp for top and bottom(no crossover is necessary)
other ways are just for another toy purpose seriously.
The best result of achieving mix of tube and solid state is to have a tube preamp to your SIM.
the only good way i see your objective is to have another type of toy.