Agreed it is easier to do with a signal an octave lower than the speaker was designed to handle. Maybe when TC made the RH750 and matching cab they thought that they might be used together?
As I said, all of what you are saying is true for other amps and other speakers, and I have no questions on that. The RH750 has Active power management (that it is often slagged off for) which means it will never produce more than 35V (see attached) without clipping (as it peaks), and it has (according to talk bass where it was tested - I have no other sources) a 2nd order HPF starting at 70Hz at 12dB/Octave.
It is incapable of producing a 'signal an octave lower than the speaker was designed to handle' as literally the speaker was designed to handle the output of the RH750.
Add as a caveat to that, the HPF is only in the power amp, so if you take the output from the DI or the preamp out, and put that through an amp, then yes you really can damage it.
I don't see there is much point adding anything else to this, although seeing as the TC RH amps are very common and most of the time paired with the RH cabs, I am obviously happy to see links to all the 'my amp broke my speaker' threads that must exit.
To the OP, don't put any bass into any speaker, as apparently it is far too risky as none of them can handle it.
tc450 stats.pdf