I've done this quite successfully, in numerous setups, sometimes the amps had variable outputs......I would tune by ear, then fine tune with a rented spectrum analyzer to get it perfectly balanced, surprisingly I did pretty good by ear.
A lot a club PA pro audio systems are designed tri-amped like that. The Grateful Dead's famous "wall of sound" PA was built in this fashion. I've done it both passive and active......one system with no varible output on the amps, had an electronic crossover with volume controls for each amp. You can get the best of both worlds, SS on the bottom & tubes for the mids and highs. My current set up is factory built for that, I have big Genesis speakers, they have their own SS bass or sub amp, then I'm using tubes for the mid ribbons and tweeter ribbons.
A lot a club PA pro audio systems are designed tri-amped like that. The Grateful Dead's famous "wall of sound" PA was built in this fashion. I've done it both passive and active......one system with no varible output on the amps, had an electronic crossover with volume controls for each amp. You can get the best of both worlds, SS on the bottom & tubes for the mids and highs. My current set up is factory built for that, I have big Genesis speakers, they have their own SS bass or sub amp, then I'm using tubes for the mid ribbons and tweeter ribbons.