What if I use a cable from BH550's balanced output into the combo's effect return?
That way I could use the preamp of the TC head and the power amp of the combo, so I won't need to worry about damaging the cab, is it right?
I'm not trying to be condescending, but I think you're making this way more complicated than it needs to be. You got your answer from @Old Garage-Bander in the first thread response...
Q: Can I use a 550 watt amp with a cabinet rated as handling 250 watts, without damaging the speaker?
A: Yes, but you should listen for any signs of distress from the speaker, and back off, should you hear any.
That's pretty much it.
You have numbers and labels and you have the practical reality of using gear. Being aware of signs of distress from speakers is something that you should do regardless of concerns about power mismatches...it's not like someone should sit by with a speaker farting, distorting, the cone traveling like crazy, possibly a faint burning smell and just ignore it saying "Yeah, I see, smell and hear that, but it's okay; this is a 500 watt amp and the speaker is rated at 600 watts!"
Another practical reality of using gear is that power ratings are maximums/limits that are intended to address potential, not operating constants. Because an amp has a rated maximum of 500 watts does not mean that whenever that amp is being used, the output is 500 watts. The way that the relationship between power (in this case, wattage) and loudness works, if a given amp reaches its maximum capacity for loudness at 500 watts, at 50% of its potential loudness, the power is about 50 watts...at 75% of its potential loudness, the output is about 200 watts.
The bottom line is that it doesn't really matter what those numbers and labels say, because they don't override what you hear and see (and sometimes smell).