For subwoofers a SS amp is definitely preferred. With the lower output impedance it can better handle the resonances of the big woofer and its box and reflex port. It can hit harder and will usually be more detailed. Also, SS watts are cheap and the tonality is not as important at low frequencies, so better to have the power where it is needed in the LFs. If you cross your sub below 100Hz between SS and tubes you won't notice any tonal differences. But the normal subwoofer integration issues (phase and level) will still require fine tuning by ear or measurement regardless of the type of amp.
Rosenberg was talking about biamping across the M>T crossover, and using the same kind of amps there is a good idea, because that frequency range is themost sensitive to our ears and we can hear every flaw there. We are most sensitive to discombobulation at 2-4kHz than any other freq band. At subwoofer crossover freq we are sensitive to phase misalignment, but that is not related to the amp type. The easiest way to get the same amp type across the crossover is vertical biamping, where you use the same stereo amplifier on mids and tweeters of the same speaker. By using the same amps you minimize the negative effects caused by tonal differences of the amps, phase differences, impedance issues, etc. At tweeter crossover freq he is definitely correct.
Rich