If amplifiers are close to their gain, sensitivity and output power settings you may use an extra volume controller to tame gain of the higher gain amplifier.
If you're using two completely different amps such as 30W tube and kilowatt SS amp than you'll definitely need an active crossover and remove the built-in one from your speakers.
If you're using two completely different amps such as 30W tube and kilowatt SS amp than you'll definitely need an active crossover and remove the built-in one from your speakers.